MOLECULAR ANALYSIS OF SAR1-RELATED CDNAS FROM A MOUSE PITUITARY CELL-LINE

Vesicular transport between the endoplasmic reticulum (ER) and the Gol gi in the yeast Saccharomyces cerevisiae requires a Ras-like, small GT P-binding protein, Sarlp[1-3]. Whether a functional homologue operates in export from the ER in mammalian cells is unknown, nor is it clear if transport in other branches of the secretory pathway requires membe r(s) of a gene family. In this study, we used a PCR approach to examin e the complexity of SARI-related sequences expressed in mammalian cell s that possess multiple secretory pathways. Amplification of cDNA sequ ences from rodent pituitary cells with primers corresponding to two co nserved GTP binding domains of Sarlp yielded several clones with seque nces homologous to Sari and/or the closely related ADP-ribosylation fa ctor (ARF) family. Of these, only two showed closer homologies to S. c erevisiae Sari than members of the ARF family and are designated as mS ARa and mSARb. Northern blot analysis shows that mSARa is expressed in most tissues including liver, heart, brain, skeletal muscle and kidne y. In contrast, mSARb is preferentially expressed in skeletal muscle a nd liver. The full-length cDNA of mSARa isolated from a mouse pituitar y AtT-20 cDNA library encodes a protein of 198 amino acids, and is 61. 6% identical to Sarlp from S. cerevisiae. Thus in contrast to the larg e rab family of GTP-binding proteins, vesicular transport in mammalian cells appears to be mediated by a relatively small number of Sari-rel ated proteins.