Characterization of a new class of nuclear localization signals

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The nucleus of animal and human cells is a vitally important cellular organelle that contains all our genes and the machinery that regulates them. Controlled gene expression, i.e. the regulated transfer of information from our genetic blueprint, the DNA, to the molecular "assembly lines" that build up our proteins is indispensable for all cellular function, including, growth, proliferation, metabolism and movement. Information transfer between the nucleus and other parts of the cells requires the continuous and subtly controlled entry and exit of proteins into and out of the nucleus.  Of the approx. 20,000 different proteins of the human body, almost one third (>6000) has the capacity to localize to the nucleus. Entry and exit - also called nuclear import and export - occurs via specialized gates, the nuclear pore (NP). This large protein complex acts both as a guide and a barrier, thereby controlling the shuttling of proteins across the nuclear membrane. Understanding this traffic, which is essential for all normal cell functions, is a fundamentally important aspect of cell biology. How does the NP recognize which molecules are to be transported (i.e. which is the "cargo") and which direction they should go (import or export)? Cargo molecules contain "molecular postal codes" or so called nuclear localization and nuclear export signals (NLS and NES). These molecular segments interact with elements of the transport machinery. Surprisingly, however, such postal codes are unknown in 60% of shuttling proteins. During our previous work on the regulation of a growth-promoting protein, we identified a new sequence, which has the capacity to mediate nuclear import. This is a unique postal code, which vastly differs from all the known NLS. We also discovered that import of this new signal is regulated by mechanical factors, acting via the cell's skeleton. Importantly, we also found similar molecular signatures (variant of the sequence) in a whole set of other, unrelated proteins, which are all known to control gene expression. These proteins play key role in variety of functions, including lipid metabolism, cell growth and immune defense. However we do not know if these variants of the discovered sequence can bring about import and affect protein function. Based on our results, we hypothesize that this molecular signature represents a new class of NLS. The objective of our research program is to characterize this NLS. We wish to identify the critical features necessary for its efficient nuclear entry, the mechanism and regulation of its import and its role in the localization and function of the proteins that contain it. We have started building a whole new molecular toolkit, which will allow us to monitor the nuclear traffic and function of the new NLS. These studies therefore have the capacity to unravel an entirely new class of nuclear localization signals, major step in understanding nuclear traffic, a fundamental process in all vital functions.