Functional characterization of Hat trick a novel modulator of Notch signaling in Drosophila melanogaster

Abstract

The growth of a single cell into multicellular organism is regulated by a few key signaling pathways. The ability of these basic signaling pathways, to regulate bewildering array of cellular and developmental processes, is achieved by a massive crosstalk that occur between them and by context specific interactions with other proteins. Further, these pathways are strictly regulated at multiple steps to fine tune the signaling outcome in a specific cellular context. Evolutionarily conserved Notch pathway is one such widely used intercellular communication systems that regulate proper growth and development of multicellular organisms. The Notch mutant was isolated more than a century ago as a dominant X-linked mutation that exhibits notches in wing margin phenotype in Drosophila melanogaster and hence derived its name Notch (Mohr 1919, Morgan and Bridges, 1916). Several genetic, molecular, biochemical and bioinformatic approaches have been used to unravel the components of Notch circuit. Pleiotropic Notch functions in dose dependent manner to regulate different developmental processes such as cell fate determination, differentiation, proliferation, apoptosis, stem cell maintenance etc. One of the most unique properties of Notch signaling is that it is highly pleiotropic and the signaling output is depends on different developmental and cellular contexts. Decades after its discovery, Notch gene was cloned in Drosophila and molecular characterization revealed that the gene encodes a transmembrane receptor. The Notch receptor is translated as a 300 kDa polypeptide and during its maturation in trans-Golgi network, full length protein is proteolytically cleaved by Furin-like convertases (S1 cleavage) which give rise to a 180 kDa N-terminal extracellular subunit (NEC) and a 120 kDa C-terminal transmembrane intracellular subunit (NTM) (Blaumueller et al. 1997). This heterodimeric Notch receptor is translocated to the cell membrane where it interacts with ligands of the DSL family (Delta and Serrate/Jagged in Drosophila and mammals and LAG-2 in C. elegans) from the neighboring cell. Binding of ligands to extracellular domain leads to second proteolytic cleavage (S2) by ADAM family of metalloproteases (Brou et al. 2000). This is followed by an intramembrane cleavage (S3) by and#947;-secretase complex (Presenilin, Nicastrin, PEN-2 and APH-1). newline