Why do cellular proteins linked to K63-polyubiquitin chains not associate with proteasomes?

Area of Science:

• Cellular Biology
• Protein Degradation
• Ubiquitination

Background:

• Ubiquitination regulates protein fate, with K48-linked chains targeting proteins to proteasomes and K63-linked chains directing them to lysosomes.
• Despite distinct cellular destinations, purified proteasomes degrade both K48- and K63-ubiquitinated substrates similarly, indicating a regulatory mechanism in vivo.

Purpose of the Study:

• To investigate the cellular mechanisms preventing proteasomal degradation of K63-ubiquitinated proteins.
• To identify factors responsible for the selective targeting of ubiquitinated proteins to either proteasomal or lysosomal degradation pathways.

Main Methods:

• Affinity purification using ubiquitinated proteins as ligands to identify interacting factors.
• Mammalian cell culture and knockdown experiments (ESCRT0) to assess in vivo function.
• Analysis of protein-ubiquitin chain interactions with proteasomes.

Main Results:

• Soluble cellular factors selectively bind K63-ubiquitin chains, inhibiting proteasome association.
• ESCRT0 components (STAM, Hrs) were identified as key factors binding K63 chains and blocking proteasomal degradation.
• Rad23 proteins, particularly hHR23B, specifically bind K48-ubiquitin chains and promote proteasome binding.

Conclusions:

• The specificity of binding proteins like ESCRT0 for K63 chains and Rad23 for K48 chains dictates the degradation pathway.
• This selective binding mechanism ensures K63-ubiquitinated proteins are routed to the endosomal-lysosomal pathway, while K48-ubiquitinated proteins are directed to the proteasome.