The zinc finger protein NGFI-A exists in both nuclear and cytoplasmic forms in nerve growth factor-stimulated PC12 cells

Abstract

The NGFI-A gene, which encodes a zinc finger protein with a predicted molecular mass of congruent to 54 kDa, is rapidly activated in PC12 cells by nerve growth factor (NGF). As a transcription factor, NGFI-A is a potentially important mediator of the cellular response to this growth factor. To characterize this protein, antibodies to four different domains of NGFI-A were raised. Immunogens included a bacterial trpE/NGFI-A fusion protein (A310) and three peptides predicted from the NGFI-A cDNA nucleotide sequence. Three of these antisera recognize two predominant NGFI-A species in NGF-stimulated PC12 cells: a 54-kDa form and a closely spaced doublet at 84 kDa. However, one of these antisera, directed against the last 15 carboxyl terminal residues of NGFI-A, recognizes only the 84-kDa species. Both NGFI-A species are rapidly induced in PC12 cells by NGF, phorbol ester, and the calcium ionophore A23187. Pulse-chase analysis revealed no obvious precursor-product relationship between these two forms and demonstrated that both the 54- and 84-kDa species are short-lived proteins. V8 protease digestion of the 54- and 84-kDa forms resulted in the formation of several small peptides that were common to both species. The digest of each species also contained one large, relatively V8 protease-resistant fragment that was substantially larger in digests of the 84-kDa form. These two fragments contained common epitopes and were derived from the amino-terminal portion of the NGFI-A protein. When NGFI-A was incubated with alkaline phosphatase, the 84-kDa doublet resolved into a single band with slightly increased mobility, indicating that this form is phosphorylated. Cell fractionation studies demonstrated that the 84-kDa species are found exclusively in the nucleus, while the 54-kDa form resides solely in the cytoplasm. It therefore appears that the 54-kDa form lacks the signal necessary for nuclear localization and is missing a portion of the carboxyl terminal domain.