Isolation and characterization of three cDNAs coding for Rab proteins from the albumen gland of the mollusc Lymnaea stagnalis

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Abstract
Three cDNA clones encoding small GTP‐binding proteins, LS‐Rab1, LS‐Rab2 and LS‐Rab18a were isolated from a cDNA library from the albumen gland of the pulmonate snail Lymnaea stagnalis. Comparison of the deduced amino acid sequences with available sequences from the EMBL/Data Bank revealed that LS‐Rab1 and LS‐Rab2 show a sequence identity of 89–90% to the mammalian Rab1 and Rab2 proteins, and can therefore be regarded as the L. stagnalis homologs. LS‐Rab18a may be considered a new member of the Rab subfamily, closely related to mouse Rab18 (74% amino acid identity). Interestingly, LS‐Rab1 and LS‐Rab2 share a very high sequence conservation with their mammalian homologs (95–97%) over the first 178–191 N‐terminal amino acids, whereas the C‐terminal part is almost completely divergent, except for their extreme ends (2–4 amino acids). The implications of these observations for the understanding of Rab‐targeting signals are discussed. The LS‐rab cDNAs were expressed in COS‐7M6 cells. The resulting 22‐kDa products were shown to bind GTP. In the albumen gland mRNA, levels of LS‐rab1 appeared to be much higher than those of LS‐rab2 and LS‐rab18a, suggesting an important role for the LS‐Rab1 protein in the albumen gland.