Vol. 119, No. 2, 1984 March 15, 1984 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Pages 561-566 REGULATION OF THE CELLULAR ACTIN LEVELS IN RESPONSE TO CHANGES IN THE CELL DENSITY IN ATAXIA TELANGIECTASIA LYMPHOBLASTOID CELLS Peter J. McKinnon and Leigh A. Burgoyne School of Biological Sciences, Flinders University of South Australia, Bedford Park, South Australia 5042 Received January 25, 1984 Summary: The level of actJn was found to decrease markedly when ataxia telangiectasia lymphoblastoid cells were stepped from low to high density culture conditions. Additionally, as the actin levels decreased the levels of a protein species of 37K dalLons increased by orders of magnitude. Partial proteolytic digestion of the 37K protein and actin revealed that the primary structures of these proteins were not related, This phenomena was observed in three out of four ata×ia telangiectasia lymphoblastoid cell lines but not in lymphoblastoid cells derived from normal individuals. One of the most abundant cellular proteins in eukaryotes is actin. Actin genes are known to be members of a multigene family, and inLracellular actin found in nonmuscle cells consists of a heterogeneous population of actin molecules (i, 2). Six different actin molecules have been identified in mammals; B and y actin are cytoplasmic and ~ actin is found in muscle. The muscle actin can be further classified into four different subtypes (3). Polymeric actin is found as the main constituent of microfilaments, which are involved in a number of cellular functions including endocytosis, receptor internalisation and cell motility (4). A large number of cellular proteins have been shown to be able to bind to, and regulate the polymerisation and depolymerisation of actin (5, 6). This paper describes an unusual regulation of actin levels observed in lymphoblastoid cell lines derived from patients with the disorder ataxia telangiectasia (AT), which is characterized by ionising radiation sensitivity and developmental abnormalities. MATERIALS AND METHODS Cell culture: LymphoblasLoid cell lines were obtained by Epstein Barr Virus (EBV) transformation. Control lines were C5ABR, CIOABR and the AT 0006-291X/84 $1.50 Copyright © 1984byAcademic Press, Inc. 561 Allrigh~ofrepro~tctioninanyform reserved.