Members of a large family of small
GTP-binding proteins, termed Rabs in mammalian cells or Ypt and Sec4 in yeast, regulate vesicular traffic in all eukaryotic cells. These
proteins are able to bind to membranes because they are modified by the type II geranylgeranyltransferase (
GGTase-II), a multisubunit complex. Component A, encoded by the
choroideremia gene in humans, is an escort
protein that brings Rabs to component B, the catalytic alpha/beta heterodimer. Mutations in the catalytic subunits of the yeast
GGTase-II (Bet2p/Mad2p) disrupt the membrane attachment of Ypt1p and Sec4p and this in turn blocks membrane traffic. In mammalian cells, deletions in
choroideremia lead only to
retinal degeneration, even though
GGTase-II activity is defective. The yeast MRS6 gene encodes a
protein that is approximately 30% identical to the
choroideremia gene product. Here we show that the addition of recombinant Mrs6p to bacterially expressed Bet2p (beta subunit) and Mad2p (alpha subunit) reconstitutes
GGTase-II activity in vitro, demonstrating that Mrs6p is yeast component A. Like Bet2p and Mad2p, Mrs6p is required for the membrane attachment of Ypt1p and Sec4p in vivo. In contrast to what has been observed before for the loss of function of the
choroideremia gene, the depletion of Mrs6p from yeast cells blocks vesicular transport. Thus, these findings suggest that there is one essential escort
protein in yeast, while more than one may exist in mammalian cells.