CHARACTERIZATION OF MYELIN BASIC-PROTEIN CHARGE MICROHETEROGENEITY INDEVELOPING MOUSE-BRAIN AND IN THE TRANSGENIC SHIVERER MUTANT

Myelin basic protein (MBP) is a highly heterogeneous family of membran e proteins consisting of several isoforms resulting from alternative s plicing and charge isomers arising from posttranslational modification s. Although well characterized in the bovine and human species, those in the mouse are not. With the availability of a number of transgenic and knockout mice, the need to understand the chemical nature of the M BPs has become very important. To isolate and characterize the MBP spe cies in murine brain, two methods were adapted for use with the small amounts of MBP available from mice. The first was a scaled-down Versio n of the preparative CM-52 chromatographic system commonly used to iso late MBP charge isomers; the second was an alkaline-urea slab gel tech nique that required five times less material than the conventional tub e gel system and, from these gels, western blots were readily obtained . Murine MBP was resolved into two populations of charge isomers: the 18.5- and 14-kDa isoforms. Isolation and characterization of these cha rge isomers or components permitted us to assign possible posttranslat ional modifications to some of them. Component 1 (C-1), the most catio nic isomer, had a molecular weight of 14,140.38 +/- 0.79, C-2 consiste d of two 14-kDa species, 14,136.37 +/- 0.74 and 14,204.45 +/- 0.70. Tw o variants, 14,215.57 +/- 0.94 and 18,413.57 +/- 0.76, constituted C-3 . C-4, C-5, and C-8 (the least cationic isomer) each consisted of both 14- and 18.5-kDa isoforms. During myelinogenesis, the 18.5-kDa isofor m appeared first (day 4); the 14-kDa isoform appeared at day 16 and su bsequently became the dominant isoform. The transgenic shiverer mutant synthesized mainly the 18.5-kDa isoform, but none of the 14-kDa isofo rm, similar to the 4-day-old mouse. We concluded that the trangenic sh iverer was able to initiate myelinogenesis with the 18.5-kDa isoform, but was unable to complete myelinogenesis because of the absence of th e 14-kDa isoform.