Fly versus man: evolutionary impairment of nucleolar targeting affects the degradome of Drosophila's Taspase1

Area of Science:

• Biochemistry
• Molecular Biology
• Evolutionary Biology

Background:

• Human Taspase1 is crucial for development and cancer, processing key regulators like mixed-lineage leukemia protein.
• Drosophila Taspase1 (dTaspase1) cleaves trithorax, indicating functional coevolution with its human ortholog.

Purpose of the Study:

• To investigate novel mechanisms regulating protease function by analyzing dTaspase1 and comparing it to its human counterpart.
• To understand the role of localization in protease substrate specificity and evolutionary divergence.

Main Methods:

• Functional analysis of dTaspase1, including alanine scanning mutagenesis.
• Cell-based assays to determine substrate cleavage motifs and localization.
• Comparative analysis of dTaspase1 and human Taspase1, including genetic modification to introduce the NoLS domain.

Main Results:

• dTaspase1 possesses a distinct cleavage motif (Q(3)[F/I/L/M](2)D(1)↓G(1')X(2')X(3')) compared to human Taspase1 (Q(3)[F,I,L,V](2)D(1)↓G(1')x(2')D(3')D(4')), predicting a broader substrate range in Drosophila.
• dTaspase1 lacks nuclear/nucleolar localization due to the absence of the conserved NoLS domain, impacting its interaction with nuclear proteins and cleavage of nuclear substrates like upstream stimulatory factor 2.
• Genetic introduction of the NoLS domain into dTaspase1 restored nucleolar localization, nucleophosmin1 interaction, and efficient cleavage of nuclear substrates.

Conclusions:

• Protease function can diverge evolutionarily through transport and localization mechanisms, as demonstrated by the differences between dTaspase1 and human Taspase1.
• The NoLS domain plays a critical role in Taspase1 localization and substrate specificity, highlighting a mechanism for functional divergence between vertebrates and invertebrates.