Cytoskeletal functions of cytoplasmic contractile proteins

Abstract

This is a review of the evidence that the cytoplasmic contractile proteins function as a cytoskeletal system in the cytoplasmic matrix. Biochemical experiments show that cycoplasmic actin filaments can form a solid gel under conditions likely to exist in living cells. The actin filaments are associated with other proteins which amy stabilize the gel and which are involved with motile force generation like myosin. Ultrastructural studies show that actin filaments are difficult to preserve, but that under stabilizing conditions networks of actin filaments are found throughtout the cytoplasmic matrix.

Viscometric analysis of the gelation of Acanthamoeba extracts and purification of two gelation factors.

“…We confirm previous qualitative evidence (18,19) that gelation of Acanthamoeba extract requires Mg`and ATP, is reversed by cold, and is inhibited by cytochalasin B . New findings include evidence for an Mg"-and ATP-requiring "potentiation reaction" that precedes gelation and for inhibition of gelation by micromolar Ca" .…”

“…The soluble proteins extracted from a variety of cells in the cold form a solid gel when warmed to room temperature under physiological conditions (5,12,13,18,19,20,25,27,28,31) . This so-called gelation reaction has attracted considerable attention, because analysis of it promises to reveal how cells regulate the consistency of their cytoplasm.…”

“…Either oftwo purified or any one of four partially purified Acanthamoeba proteins will cross-link purified actin to form a gel, but none can account for the dependence of the reaction in the crude extract on Mg-ATP or its regulation by Ca" . This suggests that the extract contains, in addition to actin-cross-linking proteins, factors dependent on Mg-ATP and Ca" that regulate the gelation process .The soluble proteins extracted from a variety of cells in the cold form a solid gel when warmed to room temperature under physiological conditions (5,12,13,18,19,20,25,27,28,31) . This so-called gelation reaction has attracted considerable attention, because analysis of it promises to reveal how cells regulate the consistency of their cytoplasm.…”

Effects of myosin and heavy meromyosin on actin-related gelation of HeLa cell extracts.

“…While jelled extracts of Acanthamoeba (22,23), macrophages (28,29), and leukemic cells (4) shrink spontaneously, neither jelled extracts of sea urchin eggs (15,16) nor those of HeLa cells (this report) will do so. Why is a myosin supplement necessary for HeLa cell extracts to contract?…”

An abundant ubiquitous glycoprotein (GP100) in nucleated mammalian cells

“…However, many of its known properties suggest that it might play a structural role in cells. Thus most of the other abundant proteins in cells, e.g., actin [12] and tubulin [ 131, are involved in the formation and maintenance of some type of intracellular structure. It is generally assumed that the high degree of sequence conservation they manifest reflects their need to interact with other proteins when they perform a scaffolding function in cells.…”