The C-terminal domain of protein 4.1G was identified to interact with the cytosolic tail of the high affinity IgG receptor, Fc gamma RI, in yeast two-hybrid screens. Proteins of the 4.1 family have previously been found to mediate receptor/cytoskeleton interactions. In the study presented here, we show an alternatively spliced 4.1G product to be associated with increased Fc gamma RI binding in yeast two-hybrid assays, and to be selectively enriched in most immune cells at the transcript level. In addition, a detailed analysis of the 4.1G 'docking site' within Fc gamma RI is provided by examining Fc gamma RI-CY-truncated and alanine-substituted mutants. These pointed to an Fc gamma RI membrane-proximal core motif of HxxBxxxBB (H represents hydrophobic residues, B basic residues and x represents any residue), followed by hydrophobic and (potentially) negatively charged residues to be central for interaction with protein 4.1G.