Differentiation 12, 39-45 (1978) Differentiation z 0 Springer-Verlag zyx 107s Changes in Intramembranous Particle Topography and Concanavalin A Receptor Mobility Associated with Myoblast Differentiation zyxw L. zyxwvutsrq T. FURCHT' and G. WENDELSCHAFER-CRABB Depment of Laboratory Medicine and Pathology, University of Minnesota, Medical School, zyxwv Box 609 Mayo, Minneapolis, Minnesota 55455, USA These studies have examined the distribution of plasma membrane intramembranous particles (PMP) visualized by freeze fracture and concanavalin A receptors seen by ultrastructural cytochem- istry of differentiated and undifferentiated L6 myoblasts. Undifferentiated mononucleated cells have a clustered distribution of PMP on the majority of the fracture faces. Associated with cell differentia- tion and cell fusion a more uniform distribution of PMP is observed. Changes also occur with myoblast differentiation in the topography and dynamics of receptors bound to concanavalin A. If undifferentiated or differentiated cells are fured with glutaraldehyde and then reacted with con-A a uniform distribution of con-A is seen on the cell surfaces. In contrast to this if unfiied live cells are reacted at 37O C with con-A a profound redistribution occurs on differentiated cells (greater than 99% showing redistribution) while receptors remain in a uniform array on undifferentiated cells (approximately 95% uniform distribution). In addition to the membrane binding, con-A is observed to bind to an extracellular filamentous matrix seen in high density undifferentiated cultures which then appears to be degraded with differentiation and myoblast fusion. These studies show that a number of membrane changes, both structural and dynamic occur with myoblast differentiation. Introduction The relationship between cell surface phenomena and cell proliferation in normal and transformed cells is an area of continuing interest. Studies using lectins have provided useful insights into the differences between normal and transformed cells in regard to such cell sur- face phenomena as receptor redistribution and lectin- induced cellular agglutination 11-31, though these have not been uniformly corroborated 141. Freeze fracture studies have revealed differences of the intrinsic mem- brane structure in normal and transformed cells at var- ious stages of growth 151 and numerous other studies have hypothesized a coordinate control of the cell sur- face and cytoskeletal structures. I ticle; PBS: phosphate buffered saline; con-A: concanavalin-A To whom reprint requests should be addressed Abbreviations: PMP: plasma membrane intramembranous par- We have used the L6 rat myoblast continuous cell line originally isolated by YaiTe [61 to examine cell sur- face changes accompanyingdifferentiation. In the undif- ferentiated state cells are well spread out and mononu- cleated. As the cells differentiate they undergo cell fu- sion to become multinucleated myotubes. Differentia- tion is associated with the increased synthesis of myofi- brillar proteins and myofilament formation and induc- tion of enzymes including creatine phosphokinase [7,81. In this report we examine two parameters of cell surface organization accompanying differentiation: (1) plasma membrane intramembranous particle (PMP) dis- tribution and (2) concanavalinA (con-A) receptor to- pography and redistribution. Methods Clonal isolates of L6 myoblasts were kindly supplied by Dr. David Schubert, zyxwv Salk Institute and grown as described [91. 0301 -468 1/78/00 12/0039/S 01.40