# There and back again: a cell biologist’s journey from organelles to molecules

## TL;DR

This review discusses how modern methods help understand the localization of proteins within organelles and the evolving concept of cellular compartmentalization.

## Contribution

The paper reviews how imaging and mass spectrometry methods reveal protein localization across organelles and subdomains.

## Key findings

- Modern imaging and mass spectrometry methods are key to understanding protein localization.
- The concept of organelles and compartmentalization has evolved with new discoveries.
- Focus is on proteins from mitochondria and secretory pathway organelles.

## Abstract

Eukaryotic life is defined by the presence of organelles. Organelles, in turn, were classically defined as specialized membrane-bound compartments composed of a unique set of macromolecules which support specific functions. Over the last few decades, a concerted effort into uncovering which components are present in each organelle has shaped our view of cell biology. However, despite some organelles already being visualized over 100 years ago, we are still discovering new organelle residents. Furthermore, our concept of both ‘organelles’ and ‘compartmentalization’ has evolved together with our deepening understanding in a number of fields. These include: organelle substructure and organization; the network of contact sites which interconnects all organelles; and membraneless organelles and phase-separated condensates. This review explores how image- and mass spectrometry-based methods can be used to understand the spectrum of where components are localized: from complexes, to subdomains, and whole organelles. The components we mainly focus on are proteins of the mitochondria and secretory pathway organelles.