This dataset contains LC-MS/MS data from multiple experiments investigating the N⁵-methyltetrahydromethanopterin:CoM-S-methyltransferase (Mtr) complex of Methanosarcina mazei, with quantitative analysis performed using MaxQuant. Experiment 1: Strep-Tactin-based purification of the native Mtr complex from M. mazei grown on methanol. Samples were divided into two conditions: Group 1: Purification performed under aerobic conditions Group 2: Purification performed under anaerobic conditions Each condition was analyzed in biological triplicates. The aim was to quantify the relative abundance of MtrI (Uniprot ID: Q8PUD4) compared to other Mtr subunits, particularly MtrA. Experiment 2: Similar to Experiment 1, but cells were grown on H₂/CO₂ instead of methanol. Group 1: Aerobic purification Group 2: Anaerobic purification Again, biological triplicates were performed for each condition. The objective remained the same: to quantify MtrI in both groups relative to other Mtr subunits. Experiment 3: Reciprocal pulldown of Mtr subunits using His-tagged MtrI expressed in M. mazei, purified via Ni-NTA affinity chromatography. Empty vector control purifications were included to assess specificity of interactions. Full experimental details are available in the associated manuscript (Reif-Trauttmansdorff et al., 2025, in preparation).