Nuclear LC3 and LC3-associated complexes in autophagy

Autophagy is a major degradation pathway that sequesters cytoplasmic components for degradation via the lysosome. A key player in the autophagy pathway is LC3, a ubiquitin-like protein that participates in autophagosome formation and autophagy cargo selection. Until recently, LC3 thus has been thought to function primarily in the cytoplasm. We made one of the first observations that LC3 is also localized to the nucleus in living cells, and showed that its trafficking in and out of this compartment is regulated. Based on these findings we proposed that nuclear LC3 may be physiologically important. This observation set the stage for subsequent work by others in the field showing that nuclear LC3, rather than cytoplasmic LC3, is specifically targeted to autophagosomes in response to starvation. Thus, nuclear LC3 appears to play a critical role in autophagy, a possibility highlighted by our initial findings.

As a central protein in the autophagy pathway, LC3 is known to associate with over 50 proteins. However,the nature of complexes that LC3 associates with in cells has remained unclear. Using the quantitative approaches to analyze FRAP developed in my group, we found that the soluble form of LC3 associates with high molecular weight complexes in both the nucleus and cytoplasm.We went on to investigate the stoichiometry of LC3 and the LC3-interacting protein SQSTM1 within cytoplasmic complexes through a collaborative study with Steve Vogel at the NIH that brought together FRAP, fluorescence correlation spectroscopy, homo FRET, and brightness analysis. These biophysical studies have identified completely new properties of LC3-associated complexes.

Kraft, L. J., J. Dowler, P. Manral, and A. K. Kenworthy (2016) Size, organization and dynamics of soluble SQSTM1 and LC3/SQSTM1 complexes in living cells. Autophagy 12(9):1660-74. doi: 10.1080/15548627.2016.1199299.

Kraft, L. J., P. Manral, J. Dowler, and A. K. Kenworthy (2016) Nuclear LC3 associates with slowly diffusing complexes that visit the nucleolus. Traffic 17(4): 369-99. PMCID: PMC4975375

Kraft, L. J., T. A. Nguyen, S. S. Vogel and A. K. Kenworthy (2014) Size, stoichiometry, and organization of soluble LC3-associated complexes. Autophagy 10(5): 861-77. PMCID: PMC4768459

Kraft, L. J. and A. K. Kenworthy (2012) Imaging protein complex formation in the autophagy pathway: analysis of the interaction of LC3 and Atg4B(C74A) in live cells using Forster resonance energy transfer and fluorescence recovery after photobleaching. J Biomed Opt 17(1): 011008. PMCID: PMC3880812

Drake, K. R., M. Kang and A. K. Kenworthy (2010) Nucleocytoplasmic distribution and dynamics of the autophagosome marker EGFP-LC3. PLoS ONE 5(3): e9806. PMCID: PMC2843706