Ssp1p is a lipid binding protein involved in shaping of the prospore membrane during meiosis in S. cerevisiae

Meiosis is a special form of cell division which results in the
formation of haploid cells. At the end of meiosis in yeast four new
cells, the so-called spores, are formed inside the boundaries of
the mother cell. This morphogenetic process requires de novo
formation of the cell membranes. Surprisingly it is the spindle
pole bodies (SPBs) which organize the formation of these prospore
membranes (PSMs). Membrane vesicles, probably redirected from the
secretory pathway, accumulate at the cytoplasmic face of the SPB
and fuse to form a continuous membrane system. In our lab we have
identified a protein complex which localizes to the tip of this
growing membrane, the leading edge protein coat (LEP coat). In this
thesis I demonstrate that the S. cereviaiae protein Ssp1p is a
novel component of the LEP coat. It is even the most important
constituent of the LEP coat as it recruits all other components to
this structure. I can show here that Ssp1p is a lipid binding
protein and this affinity is probably required for anchoring the
protein complex in the membrane. Ssp1p would then act like a hinge,
connecting the PSM to Ady3p, another protein of the LEP coat.
Deletion of SSP1 results in a block of sporulation. Here I show
that this is most probably due to a defect in membrane shaping. In
the deletion mutant the PSM sticks very tightly to the nuclear
envelope and closes without incorporating cytoplasm. This later on
leads to a defect in spore formation and to a loss of viability.
One possible explanation for this phenotype would be that the LEP
coat serves as a scaffold which provides some structural stability
to the membrane while it grows. The protein ring at its tip might
also keep the membrane open until all constituents required are
enclosed. In addition to its lipid binding activity it is
demonstrated that Ssp1p localizes to the plasma membrane of the bud
when it is overexpressed in mitotic cells. This is a hint that
Ssp1p might be involved in membrane vesicle targeting. Ssp1p could
either be directed to target membranes by their lipid composition
or by other interacting proteins. As overexpression of Ssp1p in
mitotic cells is toxic some interference with the secretory pathway
machinery is proposed.