Enables protein kinase binding activity. Acts upstream of or within several processes, including positive regulation of immune response; positive regulation of intracellular signal transduction; and regulation of lymphocyte activation. Located in external side of plasma membrane and immunological synapse. Human ortholog(s) of this gene implicated in multiple sclerosis and type 1 diabetes mellitus. Orthologous to human CD28 (CD28 molecule). [provided by Alliance of Genome Resources, Apr 2022]