Heat shock proteins function as molecular chaperones which help protein folding and prevent protein aggregation. My study shows that celastrol, a pharmacological compound capable of up-regulating the levels of heat shock proteins, inhibits cell death and protein aggregation caused by expanded polyglutamine containing protein, and the protective effects of celastrol are dependent on heat shock factor 1. These results suggest the potential of celastrol as a therapeutic agent in the treatment of polyglutamine diseases. Sumoylation is a protein modification which plays diverse roles in regulating the target proteins. My study shows that lamin A is a target of protein sumoylation, and two lamin A mutants associated with familial dilated cardiomyopathy, E203G and E203K, exhibit decreased sumoylation. My results also indicate that sumoylation is important for the normal localization of lamin A, and support a role for altered sumoylation in the underlying molecular mechanism of cardiomyopathies associated with the E203G/E203K lamin A mutations. In the third project, my results show that amyloid precursor protein is another target of SUMO modification, and sumoylation of amyloid precursor protein reduces the levels of amyloid β aggregates, which are the primary causative factor for Alzheimer’s disease. My results provide a new mechanism for the generation of amyloid β, and indicate the potential of up-regulating activity of the cellular sumoylation machinery as an approach against Alzheimer’s disease. My results also provide the first demonstration that SUMO E2 enzyme exists in the lumen of the endoplasmic reticulum, extending the sub-cellular reach of sumoylation to include the regulation of proteins in secretory pathways.