AtQQS, an Arabidopsis-specific orphan protein, exhibits broad-spectrum antimicrobial activity through membrane disruption.

Plant-derived antimicrobial proteins represent a promising but underexplored source of new anti-infective molecules. In this study, we purified and identified AtQQS (Qua-Quine Starch, NP_189695), a 59-amino-acid orphan protein previously known as a metabolic regulator in Arabidopsis thaliana, and demonstrated direct broad-spectrum antimicrobial activity against bacterial and fungal pathogens. An antimicrobial polypeptide, AtQQS, was purified by C18 reversed-phase high-performance liquid chromatography (RP-HPLC) from A. thaliana plants and confirmed by SDS-PAGE. Recombinant AtQQS protein inhibited the growth of a wide range of pathogens including Gram-negative bacteria, Gram-positive bacteria, yeasts, and phytopathogenic molds, with minimum inhibitory concentrations (MICs) ranging from 8 to 128 μM. The protein also suppressed biofilm formation and reduced pre-formed biofilms of both Escherichia coli and Staphylococcus aureus. Mechanistic investigation using membrane-impermeable fluorescent probes demonstrated that AtQQS exerts its antimicrobial action through membranolytic disruption of both bacterial and fungal cell membranes. AtQQS was well tolerated by normal human keratinocytes (HaCaT) and dermal fibroblasts (HDF) at concentrations that inhibited microbial growth, and it selectively reduced viability of several tumor cell lines. These findings assign a direct antimicrobial function to AtQQS, an activity not previously linked to this orphan protein, and identify it as a structurally compact scaffold for further peptide-based drug development.