TRANSIENT INTERACTIONS BETWEEN COLLAGEN-TAILED ACETYLCHOLINESTERASE AND SULFATED PROTEOGLYCANS PRIOR TO IMMOBILIZATION ON THE EXTRACELLULAR-MATRIX

Heparin is capable of solubilizing a subset of collagen-tailed (A(12)) acetylcholinesterase (AChE) molecules from skeletal muscle fibers, bu t cannot detach AChE from the synaptic basal lamina (Rossi, S. G., and Rotundo, R. L. (1993) J. Biol. Chem. 268, 19152-19159). In the presen t study, we used tissue-cultured quail myotubes to show that, like adu lt fibers, neither heparin- nor high salt-containing buffers detached AChE molecules from cell-surface clusters. Prelabeling clustered AChE molecules with anti-AChE monoclonal antibody 1A2 followed by incubatio n in heparin-containing medium showed that there was no reduction in t he number or size of preexisting AChE clusters. In contrast, incubatio n of myotubes with culture medium containing heparin for up to 4 days reversibly blocked the accumulation of new cell-surface AChE molecules without affecting the rate of AChE synthesis or assembly. Newly synth esized A(12) AChE becomes tightly attached to the extracellular matrix following externalization, However, in the presence of heparin, block ing the initial interactions between A(12) AChE and the extracellular matrix results in release of AChE into the medium with a t(1/2) of sim ilar to 3 h. Together, these results suggest that once A(12) AChE is l ocalized on the cell surface, initially attached via electrostatic int eractions, additional factors or events are responsible for its select ive and more permanent retention on the basal lamina.