# Imaging of Protein Assemblies up to 231 kDa in Tissues with Nano-DESI Mass Spectrometry

## TL;DR

This paper shows how a new mass spectrometry technique can image large protein complexes in tissues, doubling the previous size limit.

## Contribution

The study introduces a new mass spectrometer system that enables nano-DESI imaging of protein assemblies up to 231 kDa.

## Key findings

- Nano-DESI MSI was used to detect protein assemblies up to 231 kDa in mouse brain and rat kidney tissues.
- Native top-down MS identified homo- and heteromeric proteoform assemblies with endogenous cofactors.
- The technique eliminated the need for complementary methods like liquid extraction surface analysis for protein identification.

## Abstract

Understanding the distribution of proteins and their
assemblies
in tissues is a major challenge in spatial biology. Mass spectrometry
imaging (MSI) with nanospray-desorption electrospray ionization (nano-DESI)
has previously enabled detection, imaging, and identification of intact
protein complexes directly from tissues, including protein assemblies
and pathological protein–metal complexes in neurodegenerative
disease. To date, nano-DESI MSI has been most effective for lower
molecular weight (MW) complexes (<100 kDa), with an upper limit
of 113 kDa. Here, we demonstrate nano-DESI at molecular weights up
to 231 kDa, more than doubling the previous limit, by combining nano-DESI
with a new mass spectrometer system architecture designed for higher
MW
analysis. Both mouse brain and rat kidney tissues were analyzed. Importantly,
protein identification by native top-down MS was performed exclusively
by use of nano-DESI. That is, complementary techniques for protein
identification, such as liquid extraction surface analysis, were not
necessary. Both homo- and heteromeric proteoform assemblies were identified
in complex with endogenous small-molecule and metal ion cofactors.
The developments lead the way to the analysis of larger oligomeric
protein assemblies and protein complexes, cementing nano-DESI as a
tool for structural biology, and with implications for molecular pathology
and drug discovery.