Translational regulation of myelin basic protein synthesis

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Abstract
Synthesis of the mouse myelin basic proteins (MBPs) was studied in reticulocyte lysates programmed with brain mRNA in the presence or absence of brain factors. Addition of brain factors to the lysates increased the incorporation of [35S] methionine into total TCA‐precipitable protein by a factor of 6–9, and the majority of this stimulation was found to be due to initiation factors. Although brain factors increased total protein synthesis, the percentage of MBP synthesis was reduced from 4.3% of the total counts incorporated (in the absence of brain factors) to 1.4% (in their presence). Increasing the concentration of brain mRNA in the lysates also reduced the relative levels of MBP synthesis. These results suggested the MBP mRNAs, as a group, were less efficiently initiated than most brain mRNAs. An analysis of the nucleotide sequence flanking the initiator codon of the MBP mRNAs indicates the presence of a second AUG codon 5 bases upstream, immediately followed by a termination codon, which may provide a structural explanation for the poorer initiation efficiency of the MBP mRNAs. Further analysis of the synthesis of the individual MBPs in the presence or absence of inhibitors of initiation (7‐methylguanosine triphosphate and aurintricarboxylic acid) and elongation (anisomycin and emetine) indicated that the 14‐kD MBP mRNA was less efficiently translated than the other MBP mRNAs. Synthesis of the 14‐kD MBP was more strongly inhibited by the initiation inhibitors than the other MBP mRNAs, and synthesis of the 14‐kD MBP was increased relative to the other MBPs in the presence of elongation inhibitors. These results are consistent with the notion that the 14‐kD MBP mRNA is initiated less efficiently than the other MBP mRNAs.

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