BIOCHIMICAET BIOPHYSICAACTA 391 BBA 75033 CHARACTERIZATION OF THE PLASMA MEMBRANE OF MYCOPLASMA LAIDLA WII. II. MODES OF AGGREGATION OF SOLUBILIZED MEMBRANE COMPONENTS THOMAS M. TERRY*,**, DONALD M. ENGELMAN*, AND HAROLD J. MOROWITZ Department of Molecular Biophysics, Yale University, New Haven, Conn. (U.S.A .) (Received September 27th , I966) SUMMARY The process of assembly of membrane components from Mycoplasma laidlawii, previously solubilized by detergent (sodium dodecyl sulfate), is investigated under conditions of dialysis to remove the detergent and allow the introduction of different levels of Mg 2+. Material aggregated under these conditions is investigated by electron microscopy, density-gradient centrifugation, and analytical ultracentrifugation. In all cases, removal of detergent leads to the formation of lipoprotein material, but large lipoprotein aggregates are only obtained in the presence of Mg 2+. Different morphologies and density-gradient profiles are seen for this aggregated material, depending on the initial detergent concentration and on Mg 2+ concentration. Only under a limited set of conditions does the majority of this material closely resemble the original membrane. INTRODUCTION In studying the macromolecular organization of biologically ordered structures, it is often useful to disaggregate the structures into their component parts, to study the properties of these parts, and to investigate the modes of reassembly of these components under suitable conditions. Such a study of reassembly in vitro may yield information on the in vivo assembly process. Our purpose in this investigation has been to gain such information on cell membrane structure and synthesis, using macromolecular structural elements derived from completely dissolved plasma membrane. In earlier work we described the formation of membrane-like material from detergent-solubilized membrane components of M. laidlawii 1. Subsequent studies ~ have shown that the state of this solubilized material is a function of the concentration of detergent (sodium dodecyl sulfate). In addition we have found that the nature of * Part of this material will be submitted in partial fulfilment of the requirements for the Ph.D. degree at Yale University. ** Current address: Laboratoire de Biophysique, Universit6 de Gen~ve, Geneva, Switzer- land. Biochim. Biophys. Acta, 135 (1967) 391-4o 5