Two different RNA binding activities for the AU-rich element and the poly(A) sequence of the mouse neuronal protein mHuC

HuC is one of the RNA binding proteins which are suggested to play important roles in neuronal differentiation and maintenance. We cloned and sequenced cDNAs encoding a mouse protein which is homologous to human HuC (hHuC). The longest cDNA encodes a 367 amino acid protein with three RNA recognition motifs (RRMs) and displays 96% identity to hHuC. Northern blot analysis showed that two different mRNAs, of 5.3 and 4.3 kb, for mouse HuC (mHuC) are expressed specifically in brain tissue. Comparison of cDNA sequences with the corresponding genomic sequence revealed that alternative 3' splice site selection generates two closely related mHuC isoforms. Iterative in vitro RNA selection and binding analyses showed that both HuC isoforms can bind with almost identical specificity to sequences similar to the AU-rich element (ARE), which is involved in the regulation of mRNA stability. Functional domain mapping using mHuC deletion mutants showed that the first RRM binds to ARE, that the second RRM has no RNA binding activity by itself, but facilitates ARE binding by the first RRM and that the third RRM has specific binding activity for the poly(A) sequence.