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XVI.A.2.b. PSI-E
Sequences of PSI-E
Sequence Similarity Search for PSI-E
XVI.A.2.b. PSI-E
In plants PSI-E has a molecular weight of about 18 kDa. The cyanobacterial polypeptide is much smaller (for a general review see Andersen and Scheller, 1993). The PSI-E polypeptide is characterized by a high content of basic amino acids; it is highly conserved among various species, except for the N-terminal parts which in plants is 20 residues longer than in cyanobacteria. The N-terminal region is rich in proline, alanine and charged amino acids, and in barley exhibits sequence similarity to PSI-D.
PSI-E is not post-translationally processed. It is synthesized as a precursor molecule with an N-terminal transit peptide responsible for directing the precursor to the appropriate cell compartment. It is not post-translationally processed. The transit peptide of the PSI-E precursor lacks the hydrophobic region that directs proteins to the thylakoid lumen. Furthermore, hydropathy plots of PSI-E indicate the lack of hydrophobic membrane-spanning helixes. This in turn suggests a peripheral location for PSI-E, which is confirmed by dissociation of the polypeptide by treatment with chaotropes, and by degradation of the polypeptide when intact thylakoids are treated with proteases.
The peripheral location of PSI-E is also confirmed by inhibition of NADP+ reduction upon incubation of thylakoids or purified PSI with PSI-E antibodies. PSI-E along with PSI-D may also serve to protect the PSI-C polypeptide from the oxygenic environment, since the PSI-C polypeptide is very unstable in the presence of oxygen.
A query for psaE addressed to the various protein databases listed in the Biology Workbench, yielded 61 records which are depicted under Sequenced PSI-E. These sequences can be used for Sequence Similarity Searches or other manipulations using one of several routines described in the Biology Workbench or elsewhere (see AntheproW which can be downloaded via anonymous ftp from: ibcp.fr; pub/Antheprot/Windows/anthe 3_1 directory). For example a Blast Sequence Similarity Search of tobacco PSI-E addressed to the Biology Workbench (SwissProt database) is depicted below.
Sequences of PSI-E
Click on Sequences to view the sequences of PSI-E retrieved from various protein sequence databases by the Biology Workbench.
Sequence Similarity Search for PSI-E
Click on Sequence Similarity Search to view the Blast sequence similarity search results of barley PSI-E from the Biology workbench (SwissProt database).
References
- Andersen, B. and H. V. Scheller (1993). Structure, Function and Assembly of photosystem I. In: Pigment-Protein Complexes In Plastids: Synthesis and Assembly . C. Sundqvist and M. Ryberg, (eds.), p 383-417, Academic Press, New York.