We are interested in understanding the biology of the deadly human malaria parasite, Plasmodium falciparum. Malaria is a nefarious human disease that primarily afflicts the tropical and sub-tropical parts of the world, infecting about 300 to 500 million people and causing almost a million deaths each year. Most of these deaths occur in sub-Saharan Africa in children under the age of 5. We deploy a wide variety of tools to study the parasite including cellular biology, chemical biology, molecular biology and biochemistry. The goal of our research is to investigate new classes of drug targets to understand their role in parasite biology and develop better drugs against them.
We aim to understand the roles of a family of proteins called chaperones or heat shock proteins (Hsp) in allowing the parasite to establish its habitat within red blood cells. All living cells have this specialized family of proteins, such as Hsp110, Hsp70 and Hsp40, that use ingenious mechanisms to maintain cellular proteostasis, prevent protein aggregation and usher proteins to their proper cellular destinations across cellular membranes.
Chaperones and proteostasis:
P. falciparum has a complex lifecycle with two hosts: the insect vector Anopheles (sexual stages) and the human host (asexual stages). The ability to thrive within such divergent hosts requires the parasite to deal with regular exposure to temperature extremes, with the mosquito host at room temperature and the human host temperature varying between 37oC and 40oC (during febrile episodes). Heat shock stress results in global unfolding of the proteome. The proteome of P. falciparum has an abundance of amino acids repeats; asparagine rich-repeats are present in one in four proteins in the proteome. Asparagine rich-repeats promote protein aggregation to a greater extent than glutamine repeats (like those found in genes associated with human neurodegenerative diseases). This property of asparagine repeat-containing proteins combined with exposure to an unfolding stress that promotes aggregation leads to the question: How does P. falciparum deal with an aggregation-prone proteome in the face of periodic heat shock stress?
Protein transport and export:
For its growth within human erythrocytes, P. falciparum has to modify this terminally differentiated human cell to establish a suitable niche. The parasite does this via a sophisticated protein export pathway that transports proteins encoded by the parasite genome through at least three membranes into the erythrocyte cytoplasm and cell membrane. Exported proteins usually (but not always) contain a recognition motif (PEXEL or VTS) that is cleaved by an ER-resident aspartic protease, Plasmepsin V. The cleaved proteins are then secreted to a distinct compartment, the parasitophorous vacuole (PV) and a protein translocon residing in the PV membrane exports proteins in the host cell. Several parasite proteins populate the PV but are not exported and the parasite also transports proteins to specialized organelles such as the apicoplast (a chloroplast-like organelle required for isoprenoid synthesis). Given the existence of several protein transport pathways in parallel, all of which require several chaperones, What is the role of chaperones in the secretory pathway? How do they facilitate transport of proteins to various compartments in the infected red blood cell?
Beck J.R.*, Muralidharan V.*, Oksman A. and Goldberg D.E. (2014) PTEX component HSP101 mediates export of diverse malaria effectors into host erythrocytes. Nature, 511, 592-595
Muralidharan V. and Goldberg D.E. (2013) Asparagine Repeats in Plasmodium falciparum Proteins: Good for Nothing? PLoS Pathog. 9(8): e1003488 (doi: 10.1371/journal.ppat.1003488)
Muralidharan V., Oksman A., Pal P., Lindquist S. and Goldberg D. E. (2012) Plasmodium falciparum Hsp110 stabilizes the Asn repeat-rich parasite proteome during malarial fevers. Nat. Commun., 3:1310 (doi:10.1038/ncomms2306)
Muralidharan V., Oksman A., Iwamoto M., Wandless T.J. and Goldberg D. E. (2011) Aspragine repeat function in a Plasmodium falciparum protein assessed via a regulatable fluorescent affinity tag. Proc. Natl. Acad. Sci. USA, 108, 4411-4416.
Russo I., Babbit S.*, Muralidharan V.*, Butler T., Oksman A. and Goldberg D. E. (2010). Plasmepsin V Licenses Plasmodium Proteins for Export Into the Host Erythrocyte. Nature ,463, 632-636
Cho J.H., Muralidharan V., Vila-Perello M., Raleigh D.P., Muir T.W. and Palmer III A.G. (2011) Tuning Protein Autoinhibition by Domain Destabilization. Nat. Struc. Mol. Biol., 18, 550-555.
Muralidharan V.*, Dutta K.*, Cho J. H., Vila-Perello M., Raleigh D. P., Cowburn D. and Muir T. W. (2006). Solution Structure and Folding Characteristics of the C-terminal SH3 Domain of c-Crk-II. Biochemistry, 45, 8874-88
Muralidharan V. and Muir T. W. (2006). Protein Ligation: An Enabling Technology for the Biophysical Analysis of Proteins. Nat. Methods, 3, 429-438
Muralidharan V., Cho J. H., Trester-Zedlitz M., Kowalik L., Chait B. T., Raleigh D.P., and Muir T. W. (2004). Domain-specific Incorporation of Noninvasive Optical Probes into Recombinant Proteins. J. Am. Chem. Soc., 126, 14004-14012