Protein import into bacterial endosymbionts and evolving organelles

Area of Science:

• Cell Biology
• Evolutionary Biology
• Microbiology

Background:

• Bacterial endosymbionts are widespread in eukaryotes, contributing essential functions and driving organelle evolution (e.g., mitochondria, plastids).
• Host protein import is a hallmark of established organelles, distinguishing them from typical endosymbionts.
• Recent research reveals 'exceptional' cases of protein import into non-organelle bacterial endosymbionts.

Purpose of the Study:

• To review current evidence for protein import into bacterial endosymbionts.
• To explore the diversity of mechanisms and challenges in establishing protein import machineries.
• To highlight the increasing number of symbiotic partnerships that blur the endosymbiont-organelle distinction.

Main Methods:

• Literature review of studies investigating protein import in various endosymbiotic systems.
• Analysis of evidence ranging from 'recently evolved' organelles with numerous imported proteins to systems with single import candidates.
• Discussion of the evolutionary and mechanistic aspects of protein translocation across endosymbiont membranes.

Main Results:

• Evidence suggests protein import into bacterial endosymbionts is more prevalent than commonly recognized.
• Protein import mechanisms vary significantly across different endosymbiotic systems, indicating independent evolution.
• Both extensively integrated systems ('recently evolved' organelles) and simpler symbioses show evidence of host-to-endosymbiont protein transfer.

Conclusions:

• Protein import into bacterial endosymbionts is a common phenomenon, challenging the traditional definition of organelles.
• Understanding these import systems is crucial for elucidating the process of cellular integration and the evolution of organelles.
• Future research should focus on addressing open questions regarding the mechanisms and extent of protein import in diverse symbioses.