Protein Transport into the Inner Mitochondrial Membrane

TIM23 and TIM22 are the two distinct translocases that facilitate protein transport across the inner membrane. TIM23 is a cation-selective pore that recognizes proteins with N-terminal matrix targeting sequences, while TIM22 recognizes carrier proteins and integral membrane proteins lacking matrix targeting sequences. Both translocases work together to sort different classes of mitochondrial precursor proteins into appropriate membrane locations.

Translocation across TIM22 is driven by the electrochemical potential of the proton gradient across the inner membrane, without requiring ATP hydrolysis or chaperone assistance. Negative charges on the inner membrane matrix side unfold the precursor and pull positively charged sequences inward. This voltage-activated mechanism allows carrier proteins to cross the membrane efficiently using only the existing ion gradient.

Matrix Hsp70 functions as a molecular motor that pulls the emerging peptide through the TIM23 channel into the matrix. Hsp70 binds at the channel mouth after TIM44 recruits it, using ATP hydrolysis energy to extract the protein. This pulling force drives translocation and prevents the precursor from aggregating during transport across the inner membrane.

Stop-transfer sequences are internal hydrophobic regions that block the precursor's complete translocation through the TIM channel. These sequences anchor the protein by forming the transmembrane segment, while the N-terminal end is pulled into the matrix by Hsp70. The TIM complex then releases the protein laterally into the inner membrane, positioning it correctly for membrane integration.

The OXA complex, also called oxidase assembly machinery, associates with precursors to prevent peptide aggregation and facilitate spontaneous integration into the inner membrane. It processes proteins following both the conservative pathway, where precursors are first transported to the matrix then reinserted, and the stop-transfer pathway for single-spanning proteins. OXA substrates include multispanning inner membrane proteins and certain carrier proteins.

Carrier proteins contain non-cleavable presequences and multiple internal import signals, including a specific structural motif called the carrier signature adjacent to transmembrane segments. Unlike matrix-targeted proteins with cleavable N-terminal sequences, carrier proteins are recognized by TOM70 at internal signals and TIM10 and TIM12 at carrier signatures. This allows them to be transported through TIM22 without matrix targeting sequences.

The conservative pathway transports precursors with N-terminal matrix targeting sequences first into the matrix, then exports them back for inner membrane insertion via the OXA complex. The stop-transfer pathway uses internal hydrophobic sequences to block translocation, causing lateral release into the membrane. Both pathways ensure proper targeting, but they serve different protein classes and insertion requirements.