Sequence and structural rules define PY-NLS-dependent nuclear import by TRANSPORTIN 1 in plants

Inside every plant cell, a complex system of gates and roads manages the flow of information. The nucleus, a distinct compartment at the cell's center, holds the genetic instructions that guide growth and response to the environment. To function, the nucleus must receive specific proteins from the rest of the cell, but these proteins cannot simply wander in; they require a specific set of instructions, known as a nuclear localization signal, to be recognized and escorted across the nuclear boundary. For decades, scientists understood how this worked in animals and fungi, identifying a common type of signal that acts like a key for a specific lock. However, it remained unclear whether plants used the same keys or if they had evolved a completely different system to manage their own cellular traffic.

A team of researchers set out to solve this mystery by investigating whether a specific type of signal, known as a proline-tyrosine nuclear localization signal, exists in plants. This signal, characterized by a specific pair of amino acids, had been well-documented in animals and fungi but was unproven in the plant kingdom. The scientists focused their work on two model plants, tobacco and Arabidopsis, to see if this pathway operated there. They discovered that plants do indeed use this signal, but the machinery that recognizes it is unique. They identified a protein called TRANSPORTIN 1, or TRN1, as the receptor that binds to these signals. In experiments conducted in a test tube, TRN1 was shown to grab onto proteins carrying this specific signal. When the researchers removed TRN1 from the plant cells, the proteins that should have entered the nucleus instead piled up in the cytoplasm, the fluid filling the cell outside the nucleus. This confirmed that TRN1 is essential for moving these specific proteins into the nucleus.

The study went further to understand exactly how this signal looks in plants compared to animals. By analyzing the structure of proteins involved in jasmonate signaling, a process plants use to respond to stress and injury, and a protein involved in breaking down fats, the researchers found that the plant version of this signal has a distinct architecture. It features a short stretch of amino acids that forms a tight coil, followed by a longer connecting section that links to the core signal. This specific arrangement, with its unique coil and extended linker, distinguishes plant signals from the models established in other organisms. Using these newly defined rules, the team created a method to scan the entire genetic code of Arabidopsis. This search identified 179 proteins that likely contain this signal and rely on TRN1 for their transport.

These findings establish a previously unknown pathway for nuclear import in plants, revealing that they possess a specialized system for recognizing and transporting a specific class of proteins. The research clarifies that while the general concept of a signal guiding a protein to the nucleus is universal, the specific details of how that signal is built and read can vary significantly between kingdoms. By defining the precise shape of the signal and the receptor that reads it, the study provides a clear map of how plants control the movement of proteins that regulate gene expression, manage RNA, and drive development. This work does not just fill a gap in knowledge about plant biology; it offers a new framework for understanding how plants organize their internal life, suggesting that the rules governing cellular logistics are more diverse than previously thought.