Chapter 27 Membrane Synthesis and Turnover in Secretory Cell Systems

Abstract

Publisher Summary Although the idea of a partial overlapping in the composition of the various types of cellular membranes is now widely accepted, only a few multimembrane components have been adequately characterized. This chapter summarizes data regarding the in vivo and in vitro experiments on the biogenesis, membrane insertion, and turnover of one such component, the flavoprotein nicotinamide adenine dinucleotide (NADH)–cytochrome b 5 reductase located in microsomes in liver, in the Golgi complex, and in outer mitochondria1 membranes. Special emphasis is placed on the methodological aspects of these works. The relevance of findings for understanding membrane biogenesis in secretory systems is discussed. In the process of membrane biogenesis, different membrane proteins use at least two different mechanisms to become membrane-bound and to reach their final destination: cotranslational insertion, typical of some ectoproteins, and direct insertion that can occur concomitantly in several membranes without the need for transport from rough endoplasmic reticulum (ER) to other compartments. Two separate processes are believed to account for the replacement of cellular proteins and structures: autophagocytosis—by which portions of the cytoplasm are first segregated and then digested in bulk by lysosomal hydrolases—and molecular turnover.