Evidence for a glycoprotein 'signal' involved in transport between subcellular organelles. Two membrane glycoproteins encoded by murine leukemia virus reach the cell surface at different rates

We analyzed the synthesis and intracellular transport of two plasma membrane glycoproteins encoded by murine leukemia virus, an env gene-encoded glycoprotein with an apparent M(r) = 70,000 (gp70(env)) and a gag gene-encoded glycoprotein with an apparent M(r) = 93,000 (gp93(gag)). These glycoproteins were specifically isolated from the surfaces of infected cells by an extracellular antibody adsorption technique in which proteins are pulse labeled with L-[³⁵S] methionine and their arrival and turnover in plasma membranes can be kinetically studied (Krangel, M.S., Orr, H.T., and Strominger, J.L. (1979) Cell 18, 979-991). In addition, precursor and product forms of these glycoproteins were analyzed in whole cell lysates, in fractionated membranous subcellular organelles, and in released virions. Whereas gp93(gag) is transferred quantitatively into plasma membranes within 60 min of L-[³⁵S] methionine incorporation, gp70(env) is only partially transferred at that time and is still accumulating in plasma membranes after 210 min. This difference is caused by a slow step in gp70(env) processing which appears to occur in the rough endoplasmic reticulum and which precedes (within several minutes) or coincides with partial proteolytic cleavage of a larger glycoprotein precursor gPr90(env). Once past this kinetic barrier, gp70(env) is rapidly processed within 15 min via the Golgi apparatus to the plasma membranes. Transfer of env gene-encoded glycoproteins past the kinetic barrier from the precursor pool occurs randomly rather than in the linear or cohort order in which the proteins were synthesized. The differences between gp93(gag) and gp70(env) transport kinetics indicate that intracellular transport is a selective rather than passive flow process and that structural characteristics (signals) of membrane glycoproteins determine their rates of transfer. These characteristics of the transport system, and its randomness with respect to time spent in the pool, support a simple model: that transport requires binding to molecule(s) which interact with different glycoproteins with distinct affinities. Binding affinities of certain glycoproteins may be modulated by covalent alterations such as partial proteolysis or glycosylation.