Posttranslational Processing of Brain Actin

Abstract

Two short‐lived isoforms of actin, named δ‐ and ε‐actin, have been detected in brain extracts from rats labeled in vivo with [35S]methionine. These two molecular species have PI values slightly more basic than β‐ and γ‐actin, the stable isoforms of the protein found in brain tissue. Under the appropriate incubation conditions δ‐ and ε‐actin, synthesized in vivo, can be converted in vitro into β‐ and γ‐actin. This posttranslational processing of δ‐ and ε‐actin requires acetyl coenzyme A, suggesting that an acetylation step, presumably of the NH2‐terminal end, is involved in the transformation of these proteins into β‐ and γ‐actin.

Brain Actin Synthesized In Vitro Undergoes Two Different and Sequential Posttranslational Modifications

“…In this paper we show that, prior to the posttranslational modification that requires acetyl coenzyme A, actin molecules synthesized in vitro undergo a posttranslational modification that was not detected in previous studies (Garrels and Gibson, 1976; Palmer and Sabono, 1978; Garrels and Hunter, 1979; Rubenstein and Deuchler, 1979; Palmer et al, 1980). This latter modification can be detected by isoelectric focusing analysis and may correspond to a methylation mediated by Sadenosylmethionine.…”

“…In an attempt to demonstrate the synthesis of the primary translational product, and its conversion to $/eactin in the same cell-free system, the following experiment was carried out. In experiments reported previously (Palmer et al, 1980), we found that rat brain Weactin synthesized in vivo can be converted in vitro to Ply-actin, and that this conversion requires acetyl coenzyme A. If the primary translational products synthesized in vitro give rise to Weactin, this precursor-product relationship should be easily demonstrated in a pulse-chase experiment, provided that the conversion of aleactin to Plyactin is inhibited.…”

“…The incubation of the mixtures was carried out at 25°C for 60 min. In some experiments, after this incubation, aliquots of the reaction mixtures were reincubated at 30°C for 90 min, in the presence of an equal volume of the reticulocyte lysate or in the presence of an equal volume of crude or soluble brain extracts prepared as described previously (Palmer et al, 1980).…”

“…In 1976, Garrels and Gibson discovered the existence of short-lived forms of actin in tissue culture cells, and suggested that these forms, which they named 6-and eactin, were converted to mature forms of the protein as a consequence of posttranslational modifications. These short-lived forms of actin, which have PI values slightly more basic than those of the stable forms of the protein, have also been detected in rat brain tissue (Palmer and Sabono, 1978; Palmer et al, 1980). The available information on actin chemistry indicates that all the actins so far analyzed contain two covalently modified amino acid residues: a Nr-methylhistidine residue in position 73 (Asatoor and Armstrong, 1967; Johnson et al, 1967; Garrels and Gibson, 1976; Bray and Thomas, 1976; Vandekerckhove and Weber, 1978u), and a blocked NH,-terminal amino acid residue (Alving and Laki, 1966; Gaetjens and Bkany, 1966; Lu and Elzinga, 1977; Vandekerckhove and Weber, 1978u,b,c).…”

“…Recently it was shown that the conversion of the short-lived forms of actin into stable forms of the protein occurs as a posttranslational event. This posttranslational modification requires acetyl coen-zyme A, is accompanied by a shift of the PI value of the proteins towards a slightly more acidic pH, and probably corresponds to the acetylation of the NH,terminal amino acid residues of actin molecules (Garrels and Hunter, 1979; Rubenstein and Deuchler, 1979; Palmer et al, 1980).…”