Required for mitochondrial protein synthesis. Has a role in mitochondrial integrity and cell respiration. Component of the mitochondrial small ribosomal subunit. Mature mitochondrial ribosomes consist of a small (37S) and a large (54S) subunit. The 37S subunit contains at least 33 different proteins and 1 molecule of RNA (15S). The 54S subunit contains at least 45 different proteins and 1 molecule of RNA (21S) (By similarity). Belongs to the mitochondrion-specific ribosomal protein mS45 family.