cAMP protein kinase phosphorylates the Mos1 transposase and regulates its activity: evidences from mass spectrometry and biochemical analyses.

Genomic plasticity mediated by transposable elements can have a dramatic impact on genome integrity. To minimize its genotoxic effects, it is tightly regulated either by intrinsic mechanisms (linked to the element itself) or by host-mediated mechanisms. Using mass spectrometry, we show here for the first time that MOS1, the transposase driving the mobility of the mariner Mos1 element, is phosphorylated. We also show that the transposition activity of MOS1 is downregulated by protein kinase AMP cyclic-dependent phosphorylation at S170, which renders the transposase unable to promote Mos1 transposition. One step in the transposition cycle, the assembly of the paired-end complex, is specifically inhibited. At the cellular level, we provide evidence that phosphorylation at S170 prevents the active transport of the transposase into the nucleus. Our data suggest that protein kinase AMP cyclic-dependent phosphorylation may play a double role in the early stages of genome invasion by mariner elements.

Kinetic analysis of the interaction of Mos1 transposase with its inverted terminal repeats reveals new insight into the protein-DNA complex assembly.

Transposases are specific DNA-binding proteins that promote the mobility of discrete DNA segments. We used a combination of physicochemical approaches to describe the association of MOS1 (an eukaryotic transposase) with its specific target DNA, an event corresponding to the first steps of the transposition cycle. Because the kinetic constants of the reaction are still unknown, we aimed to determine them by using quartz crystal microbalance on two sources of recombinant MOS1: one produced in insect cells and the other produced in bacteria. The prokaryotic-expressed MOS1 showed no cooperativity and displayed a Kd of about 300 nM. In contrast, the eukaryotic-expressed MOS1 generated a cooperative system, with a lower Kd (∼ 2 nm). The origins of these differences were investigated by IR spectroscopy and AFM imaging. Both support the conclusion that prokaryotic- and eukaryotic-expressed MOS1 are not similarly folded, thereby resulting in differences in the early steps of transposition.

Mariner Mos1 transposase optimization by rational mutagenesis.

Mariner transposons are probably the most widespread transposable element family in animal genomes. To date, they are believed not to require species-specific host factors for transposition. Despite this, Mos1, one of the most-studied mariner elements (with Himar1), has been shown to be active in insects, but inactive in mammalian genomes. To circumvent this problem, one strategy consists of both enhancing the activity of the Mos1 transposase (MOS1), and making it insensitive to activity-altering post-translational modifications. Here, we report rational mutagenesis studies performed to obtain hyperactive and non-phosphorylable MOS1 variants. Transposition assays in bacteria have made it possible to isolate numerous hyperactive MOS1 variants. The best mutant combinations, named FETY and FET, are 60- and 800-fold more active than the wild-type MOS1 version, respectively. However, there are serious difficulties in using them, notably because they display severe cytotoxicity. On the other hand, three positions lying within the HTH motif, T88, S99, and S104 were found to be sensitive to phosphorylation. Our efforts to obtain active non-phosphorylable mutants at S99 and S104 positions were unsuccessful, as these residues, like the co-linear amino acids in their close vicinity, are critical for MOS1 activity. Even if host factors are not essential for transposition, our data demonstrate that the host machinery is essential in regulating MOS1 activity.