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Background  Lignocellulase hypersecretion has been achieved in industrial fungal workhorses such as  Trichoderma reesei , but the underlying mechanism associated with this process is not well understood. Although previous comparative genomic studies have revealed that the mutagenic  T. reesei  strain RUT-C30 harbors hundreds of mutations compared with its parental strain QM6a, how these mutations actually contribute to the hypersecretion phenotype remains to be elucidated.    Results  In this study, we systematically screened gene knockout (KO) mutants in the cellulolytic fungus  Neurospora crassa , which contains orthologs of potentially defective  T. reesei  RUT-C30 mutated genes. Of the 86 deletion mutants screened in  N. crassa , 12 exhibited lignocellulase production more than 25% higher than in the wild-type (WT) strain and 4 showed nearly 25% lower secretion. We observed that the deletion of  Ncap3m  (NCU03998), which encodes the μ subunit of the adaptor protein 3 (AP-3) complex in  N. crassa , led to the most significant increase in lignocellulase secretion under both Avicel and xylan culture conditions. Moreover, strains lacking the β subunit of the AP-3 complex, encoded by  Ncap3b  (NCU06569), had a similar phenotype to Δ Ncap3m , suggesting that the AP-3 complex is involved in lignocellulase secretion in  N. crassa . We also found that the transcriptional abundance of major lignocellulase genes in Δ Ncap3m  was maintained at a relatively higher level during the late stage of fermentation compared with the WT, which might add to the hypersecretion phenotype. Finally, we found that importation of the  T. reesei ap3m  ortholog  Trap3m  into Δ Ncap3m  can genetically restore secretion of lignocellulases to normal levels, which suggests that the effect of the AP-3 complex on lignocellulase secretion is conserved in cellulolytic ascomycetes.    Conclusions  Using the model cellulolytic fungus  N. crassa , we explored potential hypersecretion-related mutations in  T. reesei  strain RUT-C30. Through systematic genetic screening of 86 corresponding orthologous KO mutants in  N. crassa , we identified several genes, particularly those encoding the AP-3 complex that contribute to lignocellulase secretion. These findings will be useful for strain improvement in future lignocellulase and biomass-based chemical production.    Electronic supplementary material  The online version of this article (doi:10.1186/s13068-015-0302-3) contains supplementary material, which is available to authorized users.

Involvement of the adaptor protein 3 complex in lignocellulase secretion in Neurospora crassa revealed by comparative genomic screening