Belongs to an operon involved in alkylphosphonate uptake and C-P lyase. Exact function not known. PhnK is not required for the ribophosphonate triphosphate (RPnTP) synthase reaction. Forms a complex with PhnG, PhnH, PhnI and PhnJ with the suggested composition PhnG(4)H(2)I(2)J(2)K. Belongs to the ABC transporter superfamily.