Developmental regulation of Dictyostelium discoideum plasma membrane proteins

Developmental changes in the plasma membrane proteins of Dictyostelium discoideum have been studied using metabolic labeling with [35S]methionine and two-dimensional electrophoresis. Pulse labeling for 1 h at the early interphase, late interphase, aggregation, and tip formation stages of development showed that the profile of newly synthesized plasma membrane proteins changed dramatically over this interval. Only 14% of the polypeptide species were synthesized at all four stages at detectable levels; 86% of the species changed over this developmental interval according to the criterion that they were synthesized at some but not all of the four stages tested. Long-term labeling during vegetative growth followed by initiation of development showed that the "steady-state" levels of the plasma membrane proteins changed very little over the same period. The only changes were in minor species (33% overall change). Similar analyses of whole cell proteins showed 27 and 20% change, respectively. Cell surface radioiodination revealed 52 external proteins in the plasma membrane. Comparison with the uniform methionine labeling results showed that these proteins were, with one notable exception, minor membrane components. In these external proteins, also, developmental changes were limited and were observed in the less abundant species. These results demonstrate the existence of two general classes of plasma membrane proteins. The first is a population of high-abundance proteins that are present in vegetative cells and are largely conserved through development. These possibly serve "housekeeping" functions common to all stages. The second class consists of low-abundance species that are expressed in a highly stage-specific manner and which presumably participate in developmentally important functions.