Assembly factor required for Rieske Fe-S protein RIP1 incorporation into the cytochrome b-c1 (CIII) complex. Functions as a chaperone, binding to this subunit within the mitochondrial matrix and stabilizing it prior to its translocation and insertion into the late CIII dimeric intermediate within the mitochondrial inner membrane. Modulates the mitochondrial matrix zinc pool (By similarity). Interacts with RIP1. Belongs to the complex I LYR family. MZM1 subfamily.