New research from Sidney Kimmel Cancer Center at Jefferson member Gino Cingolani, PhD, recently provided critical insights into the nuclear import of key signaling proteins, including those involved in tumorigenesis.
For a vast array of cellular processes, numerous proteins must be imported and exported from the cell nucleus in a tightly regulated fashion. Typically, proteins that are delivered to the nucleus possess a specialized amino acid sequence, termed the Nuclear Localization Signal (NLS), which allows them to be recognized by adaptor proteins of the importin family. Proteins bearing a classical NLS are imported into the nucleus of human cells by the universal adaptor importin a1, which recruits the receptor importin b. However, some vital cell signaling proteins, including RCC1, NF-kB, and STAT proteins, are not efficiently transported by importin a1 and instead utilize selective import pathways mediated by dedicated importin a isoforms. The mechanisms of importin a isoform selectivity have remained obscure for more than two decades, suggesting the existence of an “importin a code” that has not yet been cracked.
In the new study, published in the journal Nature Communications, the Cingolani group defined the three-dimensional protein features that enable RCC1 to be recognized and transported by importin a3, marking a major advance in understanding the molecular basis for selective nuclear import of proteins by the dedicated importin a3 pathway. The researchers discovered that the importin a3 protein has a greater conformational flexibility than importin a1, even though both share an identical NLS-binding groove. This greater flexibility allows importin a3 to overcome obstacles in binding to NLS motifs that lie next to bulky domains of cargo proteins or that are masked by other structural features of cargo proteins, resulting in a much higher affinity for binding of these cargo proteins to importin a3 than other importin a isoforms. The study illustrated the importance of understanding subtle three-dimensional protein features and their relevance to fundamental biochemical events occurring in human cells, and it also represented the first successful attempt to decipher “the importin a code.”
The new study has implications for further elucidating the role of importin a3 cargo proteins in cancer biology and devising therapeutic approaches to treat cancer. Both RCC1 and NF-kB accumulate abnormally in the nucleus of many cancer cells, and in the case of NF-kB, this accumulation is thought to promote inflammation that can contribute to cancer. “Our work provides a framework to develop selective inhibitors of importin a3 that might inhibit or at least reduce the nuclear accumulation of RCC1 and NF-kB. Such inhibitors could selectively target cancer-promoting cargo proteins without affecting the bulk of NLS-mediated nuclear transport that uses the universal importin a1, which is required for many other normal cellular processes,” said Cingolani, who is also Professor of Biochemistry & Molecular Biology at Thomas Jefferson University.
The Cingolani laboratory is actively pursuing these promising avenues, focusing on NF-kB, investigating the structure of the NF-kB:importin a3 complex, and the cellular distribution of importin a3 in different cancer cell lines. Their ongoing studies are supported by a newly awarded R01 grant from the NIH National Institute of General Medical Sciences.