Binds and activates the KLRK1/NKG2D receptor, mediating natural killer cell cytotoxicity. {ECO:0000269|PubMed:19658097, ECO:0000269|PubMed:28559451}. This is the function of Ensembl gene identifier ENSG00000155918 (RAET1L, retinoic acid early transcript 1L).