MCS4 RNA (125 nt in length) is one of the small stable RNAs found in Mycoplasma capricolum cells. Gel shift assay was performed with the 5' end-32P-labeled MCS4 RNA and the S100 fraction from M. capricolum to identify the RNA binding proteins. Several bands were detected above free MCS4 RNA, indicating that the RNA formed complexes with proteins and/or RNAs. One of the proteins which specifically binds to MCS4 RNA was purified. The amino acid sequence of the N-terminus revealed 60 to 80% identity to those of glyceraldehyde-3-phosphate dehydrogenases (GAPDH) from various sources.