Subcellular compartments and protein topogenesis.

Abstract

A cell is surrounded by a plasma membrane. It contains various organelles, most of which are enclosed by limiting membranes. The intracellular space is thus divided into a number of subcellular compartments. Structurally, a cell is composed of membranes and the spaces enclosed by those membranes. In order to classify these compartments, the extracellular space has been designated S1 and whenever a unit membrane structure is crossed to arrive at the next space, one is added to term; the cytoplasmic space becomes S2, the intraluminal space of the endoplasmic reticulum and the intermembrane space of the mitochondria S3, and the matrix space of the mitochondria S4. Similarly, the plasma membrane is M1, the outer membrane of the mitochondria M2, and the inner counterpart M3. This classification of the subcellular compartments is useful in understanding a number of complicated cellular structures and functions. The intracellular transport of newly synthesized protein (protein topogenesis) and the probable development of subcellular organelles during phylogenesis of eukaryotic cells is discussed in terms of these subcellular compartments.