The mouse lens fiber-cell intrinsic membrane protein MP19 gene(Lim2) and granule membrane protein GMP-17 gene (Nkg7):Isolation and sequence analysis of two neighboring genes

[摘要] Purpose: The lens fiber cell intrinsic membrane protein MP19 appearsto play a key role in lens fiber cell structure or communication, andthus cataractogenesis. The goal of this study was to isolate andcharacterize the entire gene structure of the MP19 gene, termedLim2, and to investigate gene sequences surrounding thislens-specific gene.Methods: A 129/SvJ mouse genomic DNA library was screened usingradioisotope labeled bovine MP19 cDNA. From this screening, an 11 kbgenomic fragment was isolated which contained the entire Lim2 gene,and a neighboring gene, Nkg7, which codes for a 17 kDa granulocytemembrane protein termed GMP-17. The nucleotide sequence of this entirefragment was obtained using double strand automated sequencingtechniques. Using CAT and green fluorescent protein reporter constructs,Lim2 5'-upstream promoter sequences were analyzed.Results: An 11,182 base pair genomic clone containing the entiremurine Lim2 gene and another downstream gene, Nkg7, was obtainedand completely sequenced. These two genes are only 1,182 base pairsapart, from the poly(A) signal of the Lim2 gene to the publishedtranscriptional start site of Nkg7. Interestingly, the protein codedfor by Nkg7, GMP-17, is very similar to the product of the lensLim2 gene, MP19, in many respects. Both proteins are transmembraneproteins, with each having 4 transmembrane loops. The amino acidsequence of the two proteins is 34% identical, and 49% with respect tosimilar amino acids. The size of mouse Lim2 is 5,896 base pairs fromthe transcriptional start site to the poly(A) signal, and contains fiveexons and four introns. Exons 2-5 of the Lim2 gene encode apolypeptide of 173 amino acids, having over 92% identity to human MP19.Using chloramphenicol acetyltransferase (CAT) and green fluorescentprotein (GFP) reporter constructs, it was determined that about 160 bpof sequence upstream from the start of transcription is both necessaryand sufficient for efficient expression levels as well as tissuespecificity of expression.Conclusions: The mouse Lim2 gene is very similar to the humanLIM2 gene, both having the same number of exons and introns. Thecoding nucleotide sequences from both species are 88% identical, and 92%identical at the amino acid level. In the immediate 5'-upstream regionof these two genes, several highly conserved regions are observed. Dueto the similarity of the MP19 and GMP-17 proteins, it is interesting tospeculate that the lens MP19 and the lymphocyte-associated GMP-17 mayhave originated from one primordial gene which, through genetic drift,resulted in two separate proteins having similar functions in two widelyseparated tissue types.

[学科分类] 生物化学/生物物理