METABOLIC RELATIONSHIPS BETWEEN MYELIN SUBFRACTIONS: ENTRY OF PROTEINS ¹

Abstract

Abstract— Partially purified myelin from brains of 17‐day‐old rats was separated into 4 subfractions on a discontinuous sucrose gradient by virtue of heterogeneity in density and particle size. The protein composition of each subfraction was determined by densitometry following separation of proteins on polyacrylamide gels in buffers containing sodium dodecyl sulphate. The major proteins studied included two basic proteins, proteolipid protein, the major high molecular weight protein (W) and a group of high molecular weight proteins. The percentage of high molecular weight proteins decreased sequentially from fraction D to A, that of the W protein remained constant, while relative amounts of the two basic proteins increased. Proteolipid protein concentration also increased as a percentage of the total protein from fraction D to B, but the uppermost fraction. A, had a markedly lower amount than fraction B. At 1 h after intracranial injection of [3H]leucine, the specific radioactivity of the basic and proteolipid proteins decreased from fraction D to B, with proteolipid protein in fraction A again anomalous (specific radioactivity higher than expected). These results are consistent with (but do not prove) a precursor‐product relationship for individual proteins from denser to lighter subfractions, with the exception of myelin subfraction A. Experiments involving time staggered injections of a [14C] and later a [3H] labelled amino acid gave data which demonstrated that the W and basic proteins were added simultaneously (or with delays of much less than 20 min) to all of the subfractions, while proteolipid protein was added sequentially, from lower to upper fractions on the gradient. This double isotope technique also confirmed our previous observations that proteolipid protein shows a lag in entry into myelin compared to basic protein.

Plasma membrane of cultured oligodendrocytes: III. Relatedness to myelin

“…For example, ing similarities, are nevertheless distinct. Our findings there is evidence that MBP is synthesized close to the have confirmed previous observations obtained from immyelin compartment (Benjamins et al, 1976; Colman et munocytochemical studies; they have shed new light on al., 1982; Pereyra et al, 1983). This accords with the the relationship between the plasma membrane and finding that B3.f OLG synthesize little MBP (Yim et al, myelin; and they have also sharpened old controversies 1986) just as mature oligodendrocytes in situ do not or opened new ones.…”

Development and maturation of central nervous system myelin: Comparison of immunohistochemical localization of proteolipid protein and basic protein in myelin and oligodendrocytes

“…Therefore, it was concluded that the relative amounts ofBP and PLP are not necessarily constant in different fibers even in the mature myelin membrane. This conclusion is consistent with previous biochemical measurements that have shown a difference in the ratio ofBP and PLP in myelin subfractions separated on sucrose density gradients (13)(14)(15) and in myelin prepared from different brain regions (15)(16)(17). Other myelin-specific proteins might be expected to show similar heterogeneity in concentration.…”

Myelin

“…At short intervals after injection of radioactive precursor, the specific radioactivity of a given component is higher in fractions at the bottom of the gradient than in those at the top, suggesting that membranes in the lower fractions may be precursors of those above. This relationship has been demonstrated for sulfatide , total protein (Agrawal et al, 1974), phospholipids (Benjamins et al, 1976c), and the BP and PLP (Benjamins et al, 1976b). 3.…”

“…This fractionation has been done by centrifugation either on discontinuous sucrose density gradients (D. H. Adams and Fox, 1969;Mehl, 1972;A. W. Zimmerman et al, 1975;Fujimoto et al, 1976;Cammer et al, 1977;Benjamins et al, 1976) or on continuous gradients in a zonal rotor (Bourre et al, 1980;Matthieu et al, 1979a;Waehneldt, 1978a,b). By the latter technique, myelin preparations from the adult central nervous system (CNS) are distributed along a bell-shaped curve covering a range of sucrose concentrations of 0.5-0.9 M and centered on 0.67 M sucrose (d = 1.08) (Waehneldt, 1978a).…”

“…From the bottom to the top of the gradient, the fractions contain increasing amounts of myelin-enriched lipids (galactolipids and ethanolamine phospholipids) and proteins (Benjamins et al, , 1976bAgrawal et al, 1974;Fujimoto et al, 1976). 2.…”