The MAPK p38δ phosphorylates ORF1p on S/T-P motifs required for L1 retrotransposition. a ORF1p-WT or S/T-P mutants (200 μM), purified from E. coli, were incubated with 85 nM activated p38δ-WT (top) or the constitutively active p38δ mutant F324S (bottom) in the presence of [γ-32P]-ATP; bands on autoradiogram show 32P incorporation into ORF1p. ORF1p mutants are S18A/S27A/T203G/T213G (AAGG), S18A/S27A (AA), T203G/T213G (GG), S27A/T203G/T213G (SAGG), S18A/T203G/T213G (ASGG), S18A/S27A/T213G (AATG) and S18A/S27A/T203G (AAGT). b ORF1p-WT was incubated with activated p38δ-WT, p38δ-F324S, an inactive p38δ mutant D176A, or no kinase in reactions as described in (a). c A Coomassie-stained gel shows each ORF1p construct (approximately 100 ng) purified from E. coli.