Synthesis of Cytoskeletal and Contractile Proteins by Cultured IMR-90 Fibroblasts

Models of the assembly of cytoskeletal and contractile proteins of eukaryotic cells require quantitative information about the rates of synthesis of individual component proteins. We applied the dual isotope technique of Clark and Zak to measure the synthesis rates of cytoskeletal and contractile proteins in stationary and growing cultures of IMR-90 fibroblasts. Fibroblast proteins were labeled to equilibrium with [14C]leucine over several days, at the end of which there was a 4-h pulse with [3H]leucine. Fractional synthesis rates (percent per hour) were calculated from the 3H/14C ratio of cell protein extracts or protein purified by one- or two-dimensional polyacrylamide gel electrophoresis and the 3H/14C ratio of medium-free leucine. The average fractional synthesis rate for total, SDS- or urea-soluble; Triton-soluble; and cytoskeletal protein extracts in stationary cells each was ∼4.0%/h. The range of values for the synthesis of individual proteins from total cell extracts or cytoskeletal extracts sliced from one-dimensional gels was similar, though this range was greater than that for major proteins of Triton-soluble protein extacts. Three specific cytoskeletal proteins--actin, vimentin, and tubulin--were synthesized at similar rates that were significantly slower than the average fractional synthesis rate for total protein. Myosin, on the other hand, was synthesized faster than average. Synthesis rates were the same for β-and γ-actin and polymerized (cytoskeletal extract) vs. Triton-soluble actin. The same was true for α- and β-tubulin and two different forms of vimentin. Synthesis rates were uniformly higher in growing cells, though the same pattern of differential rates was observed as for stationary cells. Synthesis rates in growing cells were higher than the rate necessary to maintain the growth rate, even for those cytoskeletal proteins being synthesized slowly. Therefore, there appears to be some turnover of these cytoskeletal elements even during growth. We conclude that proteins in cytoskeletal extracts may have nonuniform rates of synthesis, but at least one important subclass of cytoskeletal proteins that comprise filament subunits have the same synthesis rates.