In general, a AAA ATPase acts in four stages - substrate recruitment, polypeptide initiation, polypeptide unfolding, and substrate release. Based on their substrate spectra, AAA ATPases can also be divided into three categories – promiscuous type, specific type, and orphan type. Zhejian's previous work has focused on the p97/Cdc48 ATPase, a typical promiscuous-type ATPase, and has uncovered its cryo-EM structures and molecular mechanism when processing a polyubiquitinated protein substrate. Building on this momentum, our lab will employ a combination of biochemistry, structural biology, and cell biology approaches to understand the critical functions of specific- and orphan-type ATPases in cell. Specific directions include:
Regulation of cytoskeleton dynamics and cell division
Microtubules, together with microfilaments and intermediate filaments, form the cell cytoskeleton. They are important for vesicle trafficking, cell division, and cell migration. Although the dynamics of microtubules is mainly mediated by the addition and removal of tubulin dimers at the plus and minus ends, a group of AAA ATPases – including Spastin, the Katanin family, and the Fidgetin family – can break a microtubule into two by severing at a middle point of the tubule. The severing of microtubules is crucial at the end of mitosis, when dissolution of the microtubules spanning two daughter cells is needed to allow separation of the daughter cells. These microtubule severing enzymes are specific-type ATPases, only processing alpha- and beta-tubulin molecules. But it remains not fully understood how these ATPases recognize or process tubulin molecules. In addition, mutations in the human Spastin gene can cause hereditary spastic paraplegia, a type of neurodegenerative disorders. The underlying molecular mechanism is largely unknown. Our lab aims to understand the roles of these microtubule severing enzymes both in normal cells and in disease cells.
Assembly of nuclear pore complex
Nuclear pore complex is a protein mega-complex on nuclear envelope, controlling the transport of macromolecules into and out from the nucleus. Although recent structural studies have largely advanced our understanding on the architecture of nuclear pore complex, it remains unclear how this mega-complex is assembled in cell. Notably, the nuclear pore complex undergoes cell cycle-dependent dissolution and re-assembly during cell division, just like the nuclear envelope. An orphan-type ATPase – Torsin – has been implicated in the nuclear pore complex assembly. But little is known about the underlying mechanism. Furthermore, mutations in the human Torsin gene can lead to dystonia, a neuromuscular disorder. Our lab is interested in unraveling the function of Torsin in both dividing cells and non-dividing neuron cells.
Nano-machines for targeted protein degradation
The AAA-family ATPases are a group of protein machines that have a combination of high specificity and great processivity. They have a huge potential in development of nano-machines for targeted protein degradation. Currently, the prevalent approach for targeted protein degradation employs the proteolysis-targeting chimera technology (PROTAC). This method hijacks the endogenous ubiquitination machinery and requires the endogenous proteasomes to clear the target proteins. In cases when the cellular ubiquitin-proteasome system is compromised, for instance, in aging cells or neurodegenerative cells, PROTAC may not be effective. Our lab aims to develop an ALPINE system (ATPase-linked protease induced elimination of targets), which would not require ubiquitination or the proteasome. This novel method, once established, would revolutionize the targeted protein degradation technology.