A gene pair from the human major histocompatibility complex encodes large proline-rich proteins with multiple repeated motifs and a single ubiquitin-like domain.

A large number of genes has been identified previously between the class I and class II gene families within the class III region of the human major histocompatibility complex. The complete sequences of two of these genes, BAT2 and BAT3 (where BAT is HLA-B-associated transcript), which are closely linked, were determined from cDNA clones. The putative BAT2 and BAT3 proteins are 228 and 110 kDa, respectively, and do not appear to be members of any known family of proteins. However, BAT3 contains an amino-terminal ubiquitin-like domain. Both BAT2 and BAT3 are very rich in proline and include short tracts of polyproline, polyglycine, and charged amino acids. In addition, these proteins contain several unrelated families of similar repeated segments. BAT2 and BAT3 are similar to other proteins with large proline-rich domains, such as some nuclear proteins, collagens, elastin, and synapsin. BAT2 also contains four Arg-Gly-Asp (RGD) motifs typical of the integrin receptor family.

此前，在人类主要组织相容性复合体（major histocompatibility complex，MHC）的III类区域内，I类与II类基因家族之间已鉴定出大量基因。研究人员从cDNA克隆中完成了其中两个紧密连锁的基因——BAT2与BAT3（其中BAT指HLA-B相关转录本（HLA-B-associated transcript））的全长序列测定。经推测，BAT2和BAT3蛋白的分子量分别为228 kDa和110 kDa，且不属于任何已知的蛋白家族。不过，BAT3的N端含有泛素样结构域（ubiquitin-like domain）。BAT2与BAT3均富含脯氨酸，且带有短片段的多脯氨酸、多甘氨酸及带电氨基酸序列。此外，这两种蛋白还包含多个来源各异的相似重复序列区段。BAT2与BAT3的结构特征与其他携带大型脯氨酸富集结构域的蛋白类似，例如部分核蛋白、胶原蛋白、弹性蛋白及突触素。BAT2还含有4个整合素受体家族典型的精氨酸-甘氨酸-天冬氨酸（Arg-Gly-Asp，RGD）基序。