Component of the PhsABC thiosulfate reductase that catalyzes the reduction of thiosulfate to sulfite and hydrogen sulfide, with menaquinol as the sole electron donor. Proton motive force (PMF) is required to drive transmembrane electron transfer within the reductase. The PhsC subunit anchors the complex to the membrane and contains the site for menaquinol oxidation. Composed of three subunits: PhsA, PhsB and PhsC. Belongs to the PhsC family.