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Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization
1Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta 30912-2100.
Insights
Researchers identified a human gene encoding a peptide transporter crucial for absorbing dietary peptides in the small intestine. This transporter, located on chromosome 13, facilitates peptide uptake but not free amino acids.
Area of Science:
- Molecular Biology
- Human Physiology
- Genetics
Background:
- Mammalian small intestine utilizes a proton (H+)-coupled peptide transporter for absorbing small peptides from digested dietary proteins.
- Previous isolation of a rabbit intestinal H+/peptide cotransporter cDNA provided a basis for further research.
Purpose of the Study:
- To identify and characterize the human homolog of the H+/peptide cotransporter.
- To investigate the functional properties and chromosomal localization of the cloned human transporter.
Main Methods:
- Screening of a human intestinal cDNA library using a probe derived from the rabbit H+/peptide cotransporter cDNA.
- Expression of the identified cDNA in HeLa cells and Xenopus laevis oocytes to assess peptide transport activity.
- Bioinformatic analysis of the predicted protein sequence and chromosomal assignment studies (somatic cell hybrid analysis, in situ hybridization).
Main Results:
- A human cDNA clone was identified, which upon expression, induced H(+)-dependent peptide transport activity.
- The encoded protein (708 amino acids) features 12 membrane-spanning domains and accepts dipeptides, tripeptides, and amino beta-lactam antibiotics, but not free amino acids.
- High homology (81% identity, 92% similarity) was observed with the rabbit ortholog, but weak homology with bacterial and yeast peptide transporters.
- The human H+/peptide cotransporter gene was localized to chromosome 13q33-->q34.
Conclusions:
- A functional human H+/peptide cotransporter has been cloned and characterized, playing a key role in intestinal peptide absorption.
- The human transporter shares significant structural and functional similarities with its rabbit counterpart but differs from microbial and yeast peptide transporters.
- The gene's localization to chromosome 13 provides a genetic marker for further studies on peptide transport and related physiological processes.
Abstract:
In mammalian small intestine, a H(+)-coupled peptide transporter is responsible for the absorption of small peptides arising from digestion of dietary proteins. Recently a cDNA clone encoding a H+/peptide cotransporter has been isolated from a rabbit intestinal cDNA library (Fei, Y.J., Kanai, Y., Nussberger, S., Ganapathy, V., Leibach, F.H., Romero, M.F., Singh, S.K., Boron, W. F., and Hediger, M. A. (1994) Nature 368, 563-566). Screening of a human intestinal cDNA library with a probe derived from the rabbit H+/peptide cotransporter cDNA resulted in the identification of a cDNA which when expressed in HeLa cells or in Xenopus laevis oocytes induced H(+)-dependent peptide transport activity. The predicted protein consists of 708 amino acids with 12 membrane-spanning domains and two putative sites for protein kinase C-dependent phosphorylation. The cDNA-induced transport process accepts dipeptides, tripeptides, and amino beta-lactam antibiotics but not free amino acids as substrates. The human H+/peptide cotransporter exhibits a high degree of homology (81% identity and 92% similarity) to the rabbit H+/peptide cotransporter. But surprisingly these transporters show only a weak homology to the H(+)-coupled peptide transport proteins present in bacteria and yeast. Chromosomal assignment studies with somatic cell hybrid analysis and in situ hybridization have located the gene encoding the cloned human H+/peptide cotransporter to chromosome 13 q33-->q34.