Adenine nucleotide-mediated subunit exchange between isoenzymes of glyceraldehyde-3-phosphate dehydrogenase.

Abstract

THE independent turnover of subunits of cellular structures such as ribosomes1 requires the exchange of subunits between associated structures and/or between the assembled unit and subunit pools. Exchange between protein components of isolated ribosomes and those of soluble cell fractions in very mild in vitro conditions has indeed been demonstrated1. Similarly, subunit exchange between isoenzymic forms of oligomeric enzymes may be involved in the control of isoenzyme levels within the cell. For example, heteromeric combinations of subunits would not necessarily be formed at the time of the initial assembly of subunits: hybrid molecules could be generated later by the exchange of subunits between homomeric forms. Subunit exchange has been observed between lactate dehydrogenase isoenzymes in mild in vitro conditions2,3 and it has been suggested that this exchange also occurs in vivo. Deal et al.4–7 studied the marked effect of adenine nucleotides on the structure of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) isolated from rabbit muscle and yeast, but their report that the ATP-mediated dissociation of these enzymes observed at 0° C does not occur appreciably at the more physiological temperature of 23° C argues against an ATP-mediated dissociation of GAPDH in vivo.