Synthesis of membrane proteins

Secretory pathway leads from biosynthesis of lipids and proteins on the ER membrane:

Endomembrane system - Wikipedia

Membranes are barriers for hydrophilic molecules and ions because of the hydrophobic core of the phospholipid bilayer. A membrane or is a two-dimensional, spherical particle separating an inside compartment from an outside compartment. In addition to lipids, a membrane contains that control the transport of hydrophilic and charged, small and large molecules into and out of the cell and intracellular organelles. The importance of the lipid bilayer membrane is its ability to function as an . This enables charge separation and thus the storage of electro-chemical energy in form of ion gradients. One example is the proton motif force (pmf or proton gradient) discussed in sections on oxidative phosphorylation and photosynthesis. Membrane proteins that serve as conductors are used by the cell to extract small quanta of this energy for synthesis or signaling mechanisms. Two examples are ATP synthesis and action potentials, respectively.

Synthesis and Intracellular Transport of Aminoglycerophospholipids in Permeabilized ..

with little or no lipid synthesis occurring in the plasma membrane ..

Since in the pancreatic exocrine cell synthesis and intracellular transport of secretory proteins can be uncoupled (1), it is possible to examine separately the metabolic requirements of the latter process. To this intent, guinea pig pancreatic slices were pulse labeled with leucine-3H for 3 min and incubated post-pulse for 37 min in chase medium containing 5 x 10-4 cycloheximide and inhibitors of glycolysis, respiration, or oxidative phosphorylation. In each case, the effect on transport was assessed by measuring the amount of labeled secretory proteins found in zymogen granule fractions isolated from the corresponding slices. This assay is actually a measure of the efficiency of transport of secretory proteins from the cisternae of the rough endoplasmic reticulum (RER) to the condensing vacuoles of the Golgi complex which are recovered in the zymogen granule fraction (16). The results indicate that transport is insensitive to glycolytic inhibitors (fluoride, iodoacetate) but is blocked by respiratory inhibitors (N2, cyanide, Antimycin A) and by inhibitors of oxidative phosphorylation (dinitrophenol, oligomycin). Except for Antimycin A, the effect is reversible. Parallel radioautographic studies and cell fractionation procedures applied to microsomal subfractions have indicated that the energy-dependent step is located between the transitional elements of the RER and the small, smooth-surfaced vesicles at the periphery of the Golgi complex. Radiorespirometric data indicate that the substrates oxidized to support transport are endogenous long-chain fatty acids.

Since in the pancreatic exocrine cell synthesis and intracellular transport of secretory proteins can be uncoupled (1), it is possible to examine separately the metabolic requirements of the latter process. To this intent, guinea pig pancreatic slices were pulse labeled with leucine-3H for 3 min and incubated post-pulse for 37 min in chase medium containing 5 x 10-4 cycloheximide and inhibitors of glycolysis, respiration, or oxidative phosphorylation. In each case, the effect on transport was assessed by measuring the amount of labeled secretory proteins found in zymogen granule fractions isolated from the corresponding slices. This assay is actually a measure of the efficiency of transport of secretory proteins from the cisternae of the rough endoplasmic reticulum (RER) to the condensing vacuoles of the Golgi complex which are recovered in the zymogen granule fraction (16). The results indicate that transport is insensitive to glycolytic inhibitors (fluoride, iodoacetate) but is blocked by respiratory inhibitors (N2, cyanide, Antimycin A) and by inhibitors of oxidative phosphorylation (dinitrophenol, oligomycin). Except for Antimycin A, the effect is reversible. Parallel radioautographic studies and cell fractionation procedures applied to microsomal subfractions have indicated that the energy-dependent step is located between the transitional elements of the RER and the small, smooth-surfaced vesicles at the periphery of the Golgi complex. Radiorespirometric data indicate that the substrates oxidized to support transport are endogenous long-chain fatty acids.


and function of membrane proteins ..

N2 - An experimental system was developed to analyze the synthesis and intracellular deployment of proteins in growing cells of Tetrahymena pyriformis. There are marked differences in the rates at which radioactive proteins appear in various parts of the cell, including certain membranes, follwing [3H]leucine pulse chase labeling experiments. Proteins labeled during a 5 min exposure to the isotope require approximately 2 hr for intracellular dissemination. Polyacrylamide gel electrophoresis indicated that all individual proteins of a particular cell fraction accumulate radioactivity at equivalent rates, except in the case of cilia. Further subfractionation of purified cilia yielded 3 components: matrix, axonemes, and membranes. The latter fraction was found to have a relatively simple protein composition involving only 4 major constituents. The matrix fraction, which had the highest specific radioactivity immediately following the 3H pulse, appeared to contain new proteins being conveyed to membranes and axonemes. By 2 hr following the chase, all 3 subfractions of the cilia had nearly equivalent specific radioactivities. Radioactive membrane lipids also seem to be transported into cilia via the matrix fraction. However, ciliary membrane lipids accumulate radioactivity more slowly than do the membrane proteins, apparently because of the tendency for radioactive lipids to exchange with preexisting lipid molecules of membranes encountered during the first stages of intracellular transport. Other major differences in rates of lipid and protein labeling qualify Tetrahymena as a potentially rewarding model system for studying the process of membrane assembly.