The human MAT1 protein belongs to the cyclin-dependent kinase-activating kinase complex, which is functionally associated to the transcription/DNA repair factor TFIIH. The N-terminal region of MAT1 consists of a C3HC4 RING finger, which contributes to optimal TFIIH transcriptional activities. We report here the solution structure of the human MAT1 RING finger domain (Met(1)-Asp(65)) as determined by (1)H NMR spectroscopy. The MAT1 RING finger domain presents the expected betaalphabetabeta topology with two interleaved zinc-binding sites conserved among the RING family. However, the presence of an additional helical segment in the N-terminal part of the domain and a conserved hydrophobic central beta strand are the defining features of this new structure and more generally of the MAT1 RING finger subfamily. Comparison of electrostatic surfaces of RING finger structures shows that the RING finger domain of MAT1 presents a remarkable positively charged surface. The functional implications of these MAT1 RING finger features are discussed.