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Molecular genetics of blood coagulation factor X

University of British Columbia

Thirty thousand colonies of a bovine liver cDNA library were screened with a mixture of synthetic oligodeoxyribonucleotides coding for bovine factor X. Five positive colonies were identified, and plasmid DNA was isolated. Cleavage with restriction endonucleases showed that these plasmids (designated pBXl-5) contained inserts of 1530 bp, 770 bp, 700 bp, 1100 bp and 930 bp. DNA sequence analysis of the plasmid with the largest insert (pBXl) confirmed that bovine factor X cDNAs had been cloned. The cDNA sequence predicts that factor X is synthesized as a single chain precursor in which the light and heavy chains of plasma factor X are linked by the dipeptide Arg-Arg. The cDNA sequence also predicts that factor X is synthesized with a preproleader peptide. It is proposed that at least five specific proteolytic events occur during the conversion of preprofactor X to plasma factor Xa. A human liver cDNA library was screened by colony hybridization with a bovine factor X cDNA probe. Three of the positive plasmids contained overlapping DNA that coded for most of human factor X mRNA. A second human liver cDNA library was screened by in situ hybridization with 32P-labeled human factor X cDNA clones obtained from the first screen. Several clones were isolated that contained longer inserts. DNA sequence analysis of these clones allowed the prediction of the amino acid sequence of the precursor form of human plasma factor X. From these studies, it is predicted that human factor X is synthesized as a single polypeptide chain precursor in which the light and heavy chains of plasma factor X are linked by the tripeptide Arg-Lys-Arg. The cDNA sequence also predicts that human factor X is synthesized as a preproprotein having an aminoterminal leader peptide of 40 amino acid residues. A comparison of the amino acid sequences of human and bovine factor X shows high sequence identity around the calcium- binding regions and catalytic regions but low sequence identity around the nonfunctional regions. A human genomic phage library was screened with a human factor X cDNA as a hybridization probe. Thirty-two overlapping phage clones were isolated. Characterization of six of these clones indicates that over 32 Kbp of contiguous sequence is represented. DNA sequence and restriction map analysis shows that the factor X gene is comprised of at least 8 exons and 7 introns. No clones representing the 5' untranslated region and the prepeptide of the leader sequence were identified. Two further genomic phage libraries and two libraries specific for the 5' region of the factor X gene were screened, but no 5' end clones were obtained. Restriction enzyme mapping and Southern blot analysis indicate that thus far, the human factor X gene maps to 24 Kbp of the human genome. Comparison of the factor X gene with other vitamin K-dependent blood coagulation factor genes reveals homologous exon organization. Within the blood coagulation serine proteases factor X, factor IX, factor VII, and protein C form a closely related gene family.

Text

2010-09-28

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http://hdl.handle.net/2429/28783

Biochemistry and Molecular Biology

University of British Columbia

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