Reaction of the covalently binding probes trinitrobenzenesulphonic acid and fluorodinitrobenzene with isolated myelin-sheath preparations [proceedings].

basic protein molecule. The N-terminal portion of the polypeptide chain favours hydrophobic interactions with lipid (London et al., 1973; London & Vossenberg, 1973), whereas ionic interactions with lipid occur a t the C-terminal region (Jones & Rumsby, 1977). These structural features are considered to be important in maintaining the cytoplasmic apposition in myelin. The basic protein interacts with both opposed surfaces (Fig. I ) . The alternating arrangement for the basic protein shown in Fig. 1 arises as a result of the compaction process outlined in Fig. 2. The proteolipid protein has the properties of an intrinsic membrane component. Some labelling of this protein has been noted in certain probe studies (reviewed in Rumsby & Crang, I977), which suggests that part of the protein is exposed at the external face of the myelin lipid phase (Fig. I ) . Other high-molecular-weight proteins in myelin are probably intrinsic components and are mainly located at the external surface. The basic protein and the proteolipid protein occur in myelin from brain tissue in about equimolar proportions. The two proteins have distinctly different lipid-binding properties, and we tentatively suggest that they may associate in the membrane t o form a 1 : I structural complex which would act as a nucleus for stabilizing the lipid phase. The complete lipid-basic protein-proteolipid protein complex would form a n overall subunit [some 1 5 n m x 4 n m (150Ax40A)] which would occupy about 80% of the myelin bilayer, the remainder between each subunit being occupied by lipid that shows little affinity for either protein species and high-molecular-weight proteins.