Arthrin, a Myofibrillar Protein of Insect Flight Muscle, is an Actin-ubiquitin Conjugate

Flight muscles of some insects contain a myofibrillar protein termed arthrin, which is closely related to actin (mw 43,000). Here we demonstrate that arthrin (mw 55,000) is ubiquitinated actin. We show that in Act88FM342, a flightless Drosophila mutant wherein the Act88F actin gene specifies a glu93----lys replacement, isoelectric points of both actin III and arthrin are shifted, revealing that both are encoded by the same gene. Arthrin reacts with an anti-ubiquitin antibody, which demonstrates that its extra mass results from ubiquitin ligation. Approximately one-seventh of myofibrillar actin is stably ubiquitinated, suggesting that there may be one arthrin molecule per actin-tropomyosin-troponin cooperative unit. Arthrin formation lags several hours behind that of actin III, implying that ubiquitination coincides with some aspect of myofibril assembly.