Characterization of the Human Hair Keratin–Associated Protein 2 (KRTAP2) Gene Family

Hair keratin–associated proteins (KRTAPs) are one of the major structural components of the hair shaft. Approximately 100 KRTAP genes have been identified in humans to date, with each of the genes classified into a number of families based on their sequence homology and the nature of the repeat structures. The biophysical features of KRTAPs, however, have remained largely unknown. In this study, we investigated the characteristics of the human KRTAP2 family members at the DNA, RNA, and protein levels. We initially found that these genes had various size polymorphisms that were mainly due to differences in the length of the 3′-noncoding sequences. Reverse transcriptase–PCR experiments further detected the presence of KRTAP2 transcripts in plucked human hairs. Using indirect immunofluorescence with an anti-KRTAP2 antibody, we found that there was a predominant expression of the KRTAP2 proteins in the keratinizing zone of the human hair shaft cortex. In addition, we showed that the KRTAP2 proteins interacted with each other and preferentially bound to hair keratins, but not to epithelial keratins. Finally, we determined that the head domain of the hair keratins was essential for the affinity to KRTAP2 proteins. Our results further enhance the crucial roles of KRTAPs in hair shaft keratinization in humans.