Where is thylakoid space

Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle. Vacuoles are membrane-bound sacs that function in storage and transport. The membrane of a vacuole does not fuse with the membranes of other cellular components. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. Previously, we mentioned vacuoles as essential components of plant cells.

If you look at Figure 2b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. Have you ever noticed that if you forget to water a plant for a few days, it wilts? As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant. The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.

You can rescue wilted celery in your refrigerator using this process. Simply cut the end off the stalks and place them in a cup of water. Soon the celery will be stiff and crunchy again. Figure 2. These figures show the major organelles and other cell components of a a typical animal cell and b a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole—structures not found in animal cells.

In this article, the rationale behind this pathway complexity is discussed in relation to the properties of the substrate proteins and the evolutionary origins of the chloroplast. Transport of proteins into or across membrane bilayers occurs on a large scale in almost every type of prokaryotic and eukaryotic cell. The underlying mechanisms have attracted a great deal of attention over the last 2—3 decades, primarily because the overall process of protein translocation represents such a major feat of biochemistry Jungnickel et al.

An entire range of proteins, differing widely in size, shape and hydrophobicity, must be recognized and then transported across membranes which are usually designed to be anything but permeable. To achieve this, cells have needed to overcome three major hurdles.

First, the substrate proteins have to be recognized as being scheduled for transport. Secondly, the proteins must be quantitatively translocated across the appropriate membrane. This is important; some proteins would be very toxic, even lethal, if active on the wrong side of the membrane.

Thirdly, the entire process often has to occur without allowing significant leakage of ions at the same time. In the case of the chloroplast, an additional hurdle has to be overcome: that of intraorganellar sorting. The chloroplast is a structurally complex organelle comprising numerous distinct compartments Robinson et al. However, in the course of evolution, most of the original genes have been transferred to the nucleus and chloroplast biogenesis thus requires the import of numerous proteins from the cytosol.

Most of the abundant thylakoid proteins are synthesized in the cytosol and these proteins must therefore be transported into the organelle and directed across the soluble stromal phase to their correct destination. Recent studies on the biogenesis of thylakoid proteins have pointed to the operation of a surprising variety of mechanisms for the targeting of proteins into and across the thylakoid membrane. In this article these advances are reviewed and the unexpected pathway complexity is discussed in terms of the evolution of the pathways and the properties of the substrate proteins.

Particular interest has centred on the biogenesis of thylakoid lumen proteins because these must also be transported across the thylakoid membrane, an especially complex targeting pathway. In fact, these studies have had a greater impact than envisaged and have paved the way for new insights into the export of bacterial proteins. Initial studies on the biogenesis of thylakoid lumen proteins gave no clues as to the actual complexity involved.

The envelope transit signal functions to transport the protein into the stroma, where it is usually removed by a stromal processing peptidase SPP. A SecY homologue is also involved Laidler et al. It is now generally assumed that the thylakoidal Sec pathway will turn out to be similar in most respects to bacterial Sec pathways. Competition studies showed that this system operates in parallel with the Sec pathway and, most importantly, the choice of pathway is dictated by the type of presequence present Cline et al.

However, recent mechanistic studies have shown that this system exhibits even more unusual properties. How this impressive feat is achieved remains to be resolved. The basic mechanistic features of the targeting pathways for lumenal proteins are illustrated diagramatically in Fig. Pathways for the targeting of thylakoid lumen proteins in chloroplasts.

The envelope transit peptides of both precursors are recognized by a protein transport system in the envelope membranes which facilitates translocation into the stroma. The envelope transit signals are usually removed at this point and the resultant intermediate forms are directed along two distinct routes. After translocation, substrates on both pathways are processed to the mature forms by the thylakoidal processing peptidase. Voelker and Barkan succeeded in isolating a maize mutant in this pathway, termed hcf Voelker and Barkan, , and the sequencing of the gene encoding the Hcf protein Settles et al.

Homologues of the hcf gene are present in nearly all of the sequenced bacterial genomes as unassigned open reading frames and it is now clear that a basically similar translocase operates in these organisms. Because these cofactors appear only to be inserted in the cytoplasm, folding of the proteins has to take place at this stage and this precludes translocation of this type of protein by the Sec pathway Santini et al.

The Escherichia coli genome contains several genes that encode Hcf homologues, one of which is unlinked whereas the other is the first gene in a four gene operon. It therefore seems likely that this type of translocase is primarily used for the targeting of two types of protein: those that bind cofactors and which are thus obliged to fold prior to translocation, and those that simply fold too rapidly or tightly for the Sec system to handle.

The thylakoid membrane houses numerous integral membrane proteins and, while a proportion are synthesized within the chloroplast, it is now clear that most are imported from the cytosol. As with lumenal proteins, in vitro insertion assays have been widely used in attempts to unravel the insertion mechanisms involved. These studies have demonstrated that at least two further pathways are followed by integral membrane proteins.

This finding comes as no real surprise because there is now good evidence that SRP plays a major role in the targeting of membrane proteins in bacteria reviewed by de Gier et al. Some differences are apparent since other SRPs in E. Insertion by this route requires nucleoside triphosphates, preferably GTP Cline et al. Rather more surprising is the finding that some proteins require none of the known targeting apparatus for their insertion into thylakoids. Because insertion is not dependent on any of the known translocation apparatus, it has been proposed that these proteins insert spontaneously into the thylakoid membrane, and the role of the signal peptide may be simply to provide an additional hydrophobic region to assist insertion of the transmembrane section in the mature protein.

These distinct insertion pathways are summarized in Fig. Apparently, these have been acquired after the transfer of the genes to the nucleus and the insertion mechanism thus differs markedly in this respect. SRP has been suggested to interact preferentially with highly hydrophobic proteins but, since some such proteins insert efficiently in the complete absence of this targeting factor, it is clear that other features of these membrane proteins must dictate pathway choice.

Distinct routes for the insertion of thylakoid membrane proteins. By analogy with bacterial systems, an additional factor, FtsY, probably functions as a soluble signal peptide receptor mediating transfer to SecYEG. It is now abundantly clear that the biogenesis of thylakoids is a highly complex process involving the operation of multiple targeting pathways.

Quite possibly, more will emerge in future studies since only a fraction of the known protein complement has been analysed in any detail. In the case of the lumenal proteins, the combined biochemical and genetic approaches have led to the identification of a novel protein translocase with unprecedented properties, and the discovery of related systems in numerous bacteria has forever changed the ways in which protein transport mechanisms are viewed.

Membrane proteins likewise use a variety of pathways for their insertion into the thylakoid network, but in this case the choice of pathway is dictated by more unknown factors. Methotrexate does not block import of a DHFR fusion protein into chloroplasts. Plant Molecular Biology 24 , — Berks BC. A common export pathway for proteins binding complex redox cofactors? Molecular Microbiology 22 , — An essential component of a novel bacterial protein export system with homologues in plastids and mitochondria.

Journal of Biological Chemistry , — FEBS Letters , — Chaal B, Howe CJ. Characterization of a cDNA encoding the thylakoidal processing peptidase from Arabidopsis thaliana. EMBO Journal 14 , — A folded protein can be transported across the chloroplast envelope and thylakoid membranes. Molecular Biology of the Cell 8 , — Two lumenal proteins are transported in the absence of ATP. Multiple pathways for protein transport into or across the thylakoid membrane.

EMBO Journal 12 , — The E. FEBS Letters , 1 —4. A strong protein unfolding activity is associated with the binding of precursor chloroplast proteins to chloroplast envelopes. Plant Molecular Biology 23 , — Differences between lumen targeting domains of chloroplast transit peptides determine pathway specificity for thylakoid transport.