Yeast Kex1p is a Golgi-associated membrane protein: deletions in a cytoplasmic targeting domain result in mislocalization to the vacuolar membrane

In comparing the partitioning of Kexlp activity from various truncation mutants with that of the wild-type, two separate groups became apparent. The Kexlp-Hpa and Kexlp-Hinc truncated proteins (those that remained membrane associated; Fig. 3 ) showed the same partitioning pattern as that of wild-type, whereas the proteins Kexlp-Xho, -AS, and -AMS (soluble proteins lacking the membrane-spanning domain) formed a different pattern with a 10-fold increase in activity at the cell surface relative to that of wild type. The total activity of each protein was approximately constant within a group. Kexlp-Hpa and Kexlp-Hinc had a total activity similar to that of wild type while the soluble forms of Kexlp had ~50% of wild-type activity. Although the decrease in activity may have been a direct consequence of the mutations, it was also possible that the reduced activity was due to secretion and subsequent degradation of the truncated soluble proteins. If such degradation of the soluble forms of Kexlp-Xho, -AS, and -AMS was occurring external to the plasma membrane, then the addition of BSA to the growth medium may lessen the extent of degradation. The addition of relatively high levels of BSA did not, however, alter the levels of total activity for Kexlp-Xho, -AS, and -AMS.