Expression of a TMC6-TMC8-CIB1 heterotrimeric complex in lymphocytes is regulated by each of the components

Chuan-Jin Wu,Xing Li,Connie L Sommers,Kiyoto Kurima,Sunmee Huh,Grace Bugos,Lijin Dong,Wenmei Li,Andrew J Griffith,Lawrence E Samelson

doi:10.1074/jbc.ra120.013045

Abstract

The TMC genes encode a set of homologous transmembrane proteins whose functions are not well understood. Biallelic mutations in either TMC6 or TMC8 are detected in more than half of cases of the pre-malignant skin disease epidermodysplasia verruciformis (EV). It is controversial whether EV induced by mutations in TMC6 or TMC8 originates from keratinocyte or lymphocyte defects. Quantification of TMC6 and TMC8 RNA levels in various organs revealed that lymphoid tissues have the highest levels of expression of both genes, and custom antibodies confirmed protein expression in mouse lymphocytes. To study the function of these proteins we generated mice with targeted deletion mutant alleles of Tmc6 or Tmc8 Either TMC6 or TMC8 deficiency induced a reduction in apparent molecular weight and/or amount of the other TMC molecule. Co-immunoprecipitation experiments indicated that TMC6 and TMC8 formed a protein complex in mouse and human T cells. MS and biochemical analysis demonstrated that TMC6 and TMC8 additionally interacted with the CIB1 protein to form TMC6-TMC8-CIB1 trimers. We demonstrated that TMC6 and TMC8 regulated CIB1 levels by protecting CIB1 from ubiquitination and proteasomal degradation. Reciprocally, CIB1 was needed for stabilizing TMC6 and TMC8 levels. These results suggest why inactivating mutations in any of the three human genes leads to similar clinical presentations. We also demonstrated that TMC6 and TMC8 levels are drastically lower and the proteins are less active in regulating CIB1 in keratinocytes than in T cells. Our study suggests that defects in lymphocytes may contribute to the etiology and pathogenesis of EV.

Highlights

The discovery that mutations in the TMC1 and Tmc1 genes caused deafness in humans and mouse models, respectively, led to basic insights into the mechanism of hearing [1]
We demonstrated that TMC6 and TMC8 heterodimerize and confirmed that they form a trimeric complex with calcium and integrin-binding protein 1 (CIB1)
By RNA analysis and protein assays using custom-made antibodies, we observed that TMC6 and TMC8 are highly expressed in lymphocytes

Summary

Introduction

The discovery that mutations in the TMC1 and Tmc1 genes caused deafness in humans and mouse models, respectively, led to basic insights into the mechanism of hearing [1]. TMC6 and TMC8 proteins were not detected via Western blotting in several mouse T cell lines (data not shown), and it appears that these proteins are endogenously expressed at low levels in Jurkat cells. We verified, using Western blotting analysis, that endogenous CIB1 co-immunoprecipitated with TMC6 and TMC8 in Jurkat cells transfected to stably express these two proteins (Fig. 3A).

Results

Conclusion