Structure of a gene for a lysosomal membrane glycoprotein (LEP100). Housekeeping gene with unexpected exon organization.

Members of a recently described family of glycoproteins constitute the major protein components of the lysosomal membrane. Changes in glycosylation patterns, redistribution to the cell surface, and increased levels of expression of these proteins are associated, in at least some cases, with cell differentiation, transformation, and metastasis. To understand further the regulation of expression of these proteins and their relationships to each other, we have isolated and characterized the gene for one of these proteins, LEP100, from chicken. Two overlapping clones were isolated which contain the nine exons and eight introns of the 17-kilobase gene for LEP 100. The 5'-flanking region of the gene contains CAAT and TATAAA sequences, but these do not appear to be used as promoter elements. Further downstream are three CCAAT boxes, with no corresponding TATA boxes, which represent putative promoter elements. Multiple CCAAT boxes, the apparent lack of a TATA box, and the GC-rich composition of the 5' region support the classification of the LEP100 gene as a TATA box-lacking housekeeping gene. As further evidence of the housekeeping nature of the LEP100 gene, Northern blots of RNA from several adult and embryonic tissues (skeletal muscle, kidney, liver, heart, gizzard, and brain) revealed a single message for LEP100 of the same size (about 3 kilobases) in each tissue. The gene's introns range in size from 104 to 7200 base pairs. Exons do not represent the four disulfide-bonded loops of the protein, but instead each cysteine of each disulfide-linked pair is encoded by a separate exon. The existence of the same sized mRNA in all tissues indicates that no alternate splicing occurs. The exon organization of the LEP100 gene suggests that it may have evolved from a primordial cysteine-containing exon by gene duplication events. It is likely that the genes of the other members of this family diverged from the same ancestral gene and have a gene organization similar to that of LEP100.