Structural organization and expression of the gene for the mouse GM2 activator protein.

The GM2 activator protein is an essential component for the degradation of GM2 ganglioside by hexosaminidase A in vivo. Mutations in the human gene coding for the GM2 activator protein cause the AB variant of GM2-gangliosidosis, a condition that is clinically indistinguishable from Tay-Sachs disease. To understand better factors affecting the expression of the GM2 activator protein gene (Gm2a) in mouse tissues, we have determined its exon-intron organization and analyzed its promoter region. Gm2a is about 14 kb, has four exons, and the 5' flanking region contains a CAAT box, Sp1 binding sites, AP-1, AP-2 sites, and a pair of IRE sites. A 1.2-kb fragment upstream from the initiation codon was shown to have promoter activity in NIH 3T3 cells. Similarities between the elements present in Gm2a and Hexa promoters might in part explain their similar expression patterns in mouse tissues. The different levels of GM2 activator protein mRNA in liver, kidney, brain, and testis are not owing to the use of different transcription start sites, because a single start site was found 50 bp upstream from the initiation codon in each these tissues. Northern blot analysis demonstrated variation in the GM2 activator protein mRNA expression during mouse development. Gm2a was mapped to Chromosome (Chr) 11, where it co-segregated with Csfgm.