The endomembrane system of all eukaryotic cells consists of a collection of membrane bound compartments. Movement of materials amongst these compartments is achieved by the regulated trafficking of cargo vesicles between compartments. In the process of differentiation, higher cells re-organize their internal membranes in an enormous variety of ways; expanding or specializing certain compartments, distributing them differently in the cell, or carrying vesicles to new destinations. Though these rearrangements are often critical to the function of the differentiated cell, little is known about how these specializations are imposed on the basic pattern of the secretory pathway. Spore formation in yeast involves a cell division that requires a similar organized rearrangement of the secretory apparatus. In this instance, retargetting of secretory vesicles gives rise to a new membrane compartment, the prospore membrane. This membrane arises by the redirection of secretory vesicles away from the plasma membrane to the cell interior. Prospore membrane formation therefore serves as a model for understanding the developmentally programmed reorganization of cellular membranes. In addition to the retargetting of secretory vesicles, new genetic requirements are imposed on the fusion and trafficking of these vesicles during sporulation. SPO2O encodes a SNARE protein essential for secretory vesicle fusion in sporulating cells. In order to understand the basis for this requirement, mutations in SPO2O that interfere specifically with its ability to support vesicle fusion in sporulating cells will be isolated. These mutations will be used to isolate genetic suppressors of the SPO2O defect. The rearrangement of the secretory pathway during sporulation is coordinated with the meiotic divisions. How this coordination is established will be investigated by examining the affect of mutations in cell-cycle regulatory genes on secretory vesicle redirection as well as examining the role in sporulation of two cell cycle regulated kinases that bind directly to Spo2Op. Finally, prospore membrane formation cannot initiate without the appearance of a sporulation-specific modification on the cytoplasmic face of the spindle pole body (SPB). SPO21 encodes a protein required for prospore membrane formation and may be necessary for this modification. The model that Spo2lp is a sporulation-specific component of the SPB will be tested and the role of Spo2 Ip in SPB modification examined.
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