We describe the characterization of a novel cDNA, mbh1 (myc basic motif homolog-1), which was found during a search for candidate factors which might interact with the c-Myc oncoprotein. Embedded within the amino acid sequence encoded by mbh1 is a region distantly related to the basic/helix-loop-helix (B/HLH) DNA-binding motif and a potential nuclear localization signal. Mbh1 encodes a polypeptide structurally similar to the actin-severing proteins gelsolin and severin. Translation of mbh1 RNA in rabbit reticulocyte extracts produces an approximately 45 kd protein capable of binding actin-coupled agarose beads in vitro in a Ca2(+)-dependent manner. Antiserum raised to a trpE/mbh1 bacterial fusion protein recognizes an approximately 45 kb protein in murine 3T3 fibroblasts, suggesting that the cDNA encodes the complete Mbh1 protein. Examination of Mbh1 localization in 3T3 fibroblasts by indirect immunofluorescence reveals a larger cell population showing diffuse staining, and a smaller population exhibiting a distinct nuclear stain. Western analysis corroborates this intracellular localization and indicates that total cellular levels and localization of Mbh1 are not affected by the cell growth state. The data suggest that Mbh1 may play a role in regulating cytoplasmic and/or nuclear architecture through potential interactions with actin.