Turnover and compartmentation of gp70/p15 E in psi 2 cells

Studies on compartmentation and turnover of murine retrovirus envelope proteins

Several aspects of turnover and degradation of cell membrane proteins were studied in an NIH 3T3 cell clone expressing the env gene of Moloney murine leukemia virus ts1. Both internalization and shedding of the extracellular domain of the envelope protein gp70 occurred at the cell surface, albeit, in the case of shedding, only a very small fraction of gp70 was shed. The turnover rate of gp70 at the cell surface was similar to that of the same protein in the postendoplasmic reticulum intracellular compartment. In the presence of L-methionine methyl ester, the transmembrane domain of the envelope protein Prp15E was degraded faster than gp70.

Mechanisms of plasma membrane protein degradation: recycling proteins are degraded more rapidly than those confined to the cell surface

Plasma membrane proteins of intact mouse 3T3 fibroblasts and H4-II-E-C3 hepatoma cells were separated into two groups based on their compartmentation between the cell surface and an intracellular compartment accessible at 20 degrees C but not at 0 degrees C. One group was derivatized at 0 degrees C with sulfosuccinimidyl 2-(biotinamido)ethyl-1,3-dithiopropionate but not at 20 degrees C. The second group was derivatized at 20 degrees C as well as at 0 degrees C. Derivatized proteins were isolated from 35S-labeled cells on streptavidin/agarose and resolved by two-dimensional PAGE. With few exceptions, pulse-chase experiments revealed that those proteins confined exclusively to the cell surface turned over slowly (t1/2, greater than 75 h), while those bimodally compartmentalized between the cell surface and the 20 degrees C accessible compartment were degraded more rapidly (t1/2, less than 31 h). These observations suggest a mechanism to explain the varied metabolic stability of plasma membrane proteins in which the half-life of each protein is determined by the proportion of time spent in the endocytic compartment.